vo_opengl: don't discard buffered video on redundant resize calls

If a VO-area option changes, gl_video_resize() is called
unconditionally. This function does something even if the size does not
change (at least it discards buffered frames for interpolation), which
can lead to stutter when you keep firing option change events during
playback.

Check for an actual resize, and if nothing changes, exit early.

sub/osd.h

@@ -82,6 +82,8 @@ struct mp_osd_res {
     double display_par;
 };
 
+bool osd_res_equals(struct mp_osd_res a, struct mp_osd_res b);
+
 // 0 <= sub_bitmaps.render_index < MAX_OSD_PARTS
 #define MAX_OSD_PARTS 5