vo_gpu: make texture offset available to CHROMA hooks

Before this commit, texture offset is set after all source textures
are finalized. Which means CHROMA hooks won't be able to align with
luma planes. This could be problematic for chroma prescalers utilizing
information from luma plane.

Fix this by find the reference texture early, and set global texture
offset early.

video/out/gpu/video.c

@@ -2072,6 +2072,23 @@ static void pass_read_video(struct gl_video *p)
         }
     }
 
+    // The basic idea is we assume the rgb/luma texture is the "reference" and
+    // scale everything else to match, after all planes are finalized.
+    // We find the reference texture first, in order to maintain texture offset
+    // between hooks on different type of planes.
+    int reference_tex_num = 0;
+    for (int n = 0; n < 4; n++) {
+        switch (img[n].type) {
+        case PLANE_RGB:
+        case PLANE_XYZ:
+        case PLANE_LUMA: break;
+        default: continue;
+        }
+
+        reference_tex_num = n;
+        break;
+    }
+
     // Dispatch the hooks for all of these textures, saving and perhaps
     // modifying them in the process
     for (int n = 0; n < 4; n++) {
@@ -2086,26 +2103,18 @@ static void pass_read_video(struct gl_video *p)
         }
 
         img[n] = pass_hook(p, name, img[n], &offsets[n]);
+
+        if (reference_tex_num == n) {
+            // The reference texture is finalized now.
+            p->texture_w = img[n].w;
+            p->texture_h = img[n].h;
+            p->texture_offset = offsets[n];
+        }
     }
 
     // At this point all planes are finalized but they may not be at the
     // required size yet. Furthermore, they may have texture offsets that
-    // require realignment. For lack of something better to do, we assume
-    // the rgb/luma texture is the "reference" and scale everything else
-    // to match.
-    for (int n = 0; n < 4; n++) {
-        switch (img[n].type) {
-        case PLANE_RGB:
-        case PLANE_XYZ:
-        case PLANE_LUMA: break;
-        default: continue;
-        }
-
-        p->texture_w = img[n].w;
-        p->texture_h = img[n].h;
-        p->texture_offset = offsets[n];
-        break;
-    }
+    // require realignment.
 
     // Compute the reference rect
     struct mp_rect_f src = {0.0, 0.0, p->image_params.w, p->image_params.h};