sub: do not copy the target image if there is no OSD/subs

It's not easy to tell whether the OSD/subs are empty, or if something is
drawn. In general you have to use osd_draw() with a custom callback. If
nothing is visible, the callback is never invoked. (The actual reason
why this is so "hard" is the implementation of osd_libass.c, which
doesn't allow separating rendering and drawing of OSD elements, because
all OSD elements share the same ASS_Renderer.)

To simplify avoiding copies, make osd_draw_on_image() instead of the
caller use mp_image_make_writeable(). Introduce osd_draw_on_image_p(),
which works like osd_draw_on_image(), but gets the new image allocation
from an image pool. This is supposed to be an optimization, because it
reduces the frequency of large allocations/deallocations for image data.

The result of this is that the frequency of copies needed in conjunction
with vf_sub, screenshots, and vo_lavc (encoding) should be reduced.
vf_sub now always does true pass-through if no subs are shown.

Drop the pts check from vf_sub. This didn't make much sense.

core/screenshot.c

@@ -235,8 +235,6 @@ static char *gen_fname(screenshot_ctx *ctx, const char *file_ext)
 
 static void add_subs(struct MPContext *mpctx, struct mp_image *image)
 {
-    mp_image_make_writeable(image);
-
     int d_w = image->display_w ? image->display_w : image->w;
     int d_h = image->display_h ? image->display_h : image->h;
 

sub/sub.c

@@ -39,6 +39,8 @@
 #include "draw_bmp.h"
 #include "spudec.h"
 #include "subreader.h"
+#include "video/mp_image.h"
+#include "video/mp_image_pool.h"
 
 
 char * const sub_osd_names[]={
@@ -274,6 +276,7 @@ struct draw_on_image_closure {
     struct osd_state *osd;
     struct mp_image *dest;
     struct mp_draw_sub_backup *bk;
+    struct mp_image_pool *pool;
     bool changed;
 };
 
@@ -283,11 +286,17 @@ static void draw_on_image(void *ctx, struct sub_bitmaps *imgs)
     struct osd_state *osd = closure->osd;
     if (closure->bk)
         mp_draw_sub_backup_add(closure->bk, closure->dest, imgs);
+    if (closure->pool) {
+        mp_image_pool_make_writeable(closure->pool, closure->dest);
+    } else {
+        mp_image_make_writeable(closure->dest);
+    }
     mp_draw_sub_bitmaps(&osd->draw_cache, closure->dest, imgs);
     talloc_steal(osd, osd->draw_cache);
     closure->changed = true;
 }
 
+// Calls mp_image_make_writeable() on the dest image if something is drawn.
 // Returns whether anything was drawn.
 bool osd_draw_on_image(struct osd_state *osd, struct mp_osd_res res,
                        double video_pts, int draw_flags, struct mp_image *dest)
@@ -298,6 +307,18 @@ bool osd_draw_on_image(struct osd_state *osd, struct mp_osd_res res,
     return closure.changed;
 }
 
+// Like osd_draw_on_image(), but if dest needs to be copied to make it
+// writeable, allocate images from the given pool. (This is a minor
+// optimization to reduce "real" image sized memory allocations.)
+void osd_draw_on_image_p(struct osd_state *osd, struct mp_osd_res res,
+                         double video_pts, int draw_flags,
+                         struct mp_image_pool *pool, struct mp_image *dest)
+{
+    struct draw_on_image_closure closure = {osd, dest, .pool = pool};
+    osd_draw(osd, res, video_pts, draw_flags, mp_draw_sub_formats,
+             &draw_on_image, &closure);
+}
+
 void osd_draw_on_image_bk(struct osd_state *osd, struct mp_osd_res res,
                           double video_pts, int draw_flags,
                           struct mp_draw_sub_backup *bk, struct mp_image *dest)

sub/sub.h

@@ -222,6 +222,11 @@ struct mp_image;
 bool osd_draw_on_image(struct osd_state *osd, struct mp_osd_res res,
                        double video_pts, int draw_flags, struct mp_image *dest);
 
+struct mp_image_pool;
+void osd_draw_on_image_p(struct osd_state *osd, struct mp_osd_res res,
+                         double video_pts, int draw_flags,
+                         struct mp_image_pool *pool, struct mp_image *dest);
+
 struct mp_draw_sub_backup;
 void osd_draw_on_image_bk(struct osd_state *osd, struct mp_osd_res res,
                           double video_pts, int draw_flags,

video/filter/vf_sub.c

@@ -155,9 +155,7 @@ static struct mp_image *filter(struct vf_instance *vf, struct mp_image *mpi)
     struct vf_priv_s *priv = vf->priv;
     struct osd_state *osd = priv->osd;
 
-    if (vf->priv->opt_top_margin || vf->priv->opt_bottom_margin ||
-        !mp_image_is_writeable(mpi))
-    {
+    if (vf->priv->opt_top_margin || vf->priv->opt_bottom_margin) {
         struct mp_image *dmpi = vf_alloc_out_image(vf);
         mp_image_copy_attributes(dmpi, mpi);
         prepare_image(vf, dmpi, mpi);
@@ -167,8 +165,8 @@ static struct mp_image *filter(struct vf_instance *vf, struct mp_image *mpi)
 
     mp_image_set_colorspace_details(mpi, &priv->csp);
 
-    if (mpi->pts != MP_NOPTS_VALUE)
-        osd_draw_on_image(osd, priv->dim, mpi->pts, OSD_DRAW_SUB_FILTER, mpi);
+    osd_draw_on_image_p(osd, priv->dim, mpi->pts, OSD_DRAW_SUB_FILTER,
+                        vf->out_pool, mpi);
 
     return mpi;
 }

video/out/vo_lavc.c

@@ -487,7 +487,6 @@ static void draw_osd(struct vo *vo, struct osd_state *osd)
         };
 
         mp_image_set_colorspace_details(vc->lastimg, &vc->colorspace);
-        mp_image_make_writeable(vc->lastimg);
 
         osd_draw_on_image(osd, dim, osd->vo_pts, OSD_DRAW_SUB_ONLY, vc->lastimg);
     }