vf: add vf_rescale_dsize()

Needed because mplayer* basically tracks DAR, which makes it harder for
filters which want to keep the SAR.

The contents of this function are duplicated in a few filters, and will
be used instead of custom code as the filters are updated later.

video/filter/vf.c

@@ -30,6 +30,7 @@
 #include "core/m_option.h"
 #include "core/m_struct.h"
 
+#include "core/options.h"
 
 #include "video/img_format.h"
 #include "video/mp_image.h"
@@ -476,6 +477,20 @@ void vf_uninit_filter_chain(vf_instance_t *vf)
     }
 }
 
+// When cropping an image that had old_w/old_h/*d_width/*d_height to the new
+// size new_w/new_h, adjust *d_width/*d_height such that the new image has
+// the same pixel aspect ratio.
+void vf_rescale_dsize(struct vf_instance *vf, int *d_width, int *d_height,
+                      int old_w, int old_h, int new_w, int new_h)
+{
+    // No idea what this is about
+    if (vf->opts->screen_size_x || vf->opts->screen_size_y)
+        return;
+
+    *d_width  = *d_width  * new_w / old_w;
+    *d_height = *d_height * new_h / old_h;
+}
+
 void vf_detc_init_pts_buf(struct vf_detc_pts_buf *p)
 {
     p->inpts_prev = MP_NOPTS_VALUE;

video/filter/vf.h

@@ -141,6 +141,9 @@ int vf_config_wrapper(struct vf_instance *vf,
                       int width, int height, int d_width, int d_height,
                       unsigned int flags, unsigned int outfmt);
 
+void vf_rescale_dsize(struct vf_instance *vf, int *d_width, int *d_height,
+                      int old_w, int old_h, int new_w, int new_h);
+
 static inline int norm_qscale(int qscale, int type)
 {
     switch (type) {