vo_opengl: remove unused ra_mapped_buffer.preferred_align field

It makes no sense to have this on an already created buffer.

If anything, the ra backend would have to export this as a global value
(e.g. struct ra field), so that whatever allocates the buffer can
account for the required alignment. Since this code is in vo_opengl.c in
the first place, and since GL doesn't dictate any special alignment
here, it doesn't make sense in the first place to export this. (Maybe
something like this will be required later.)

video/out/opengl/ra.h

@@ -91,7 +91,6 @@ struct ra_mapped_buffer {
     void *priv;
     void *data;             // pointer to first usable byte
     size_t size;            // total size of the mapping, starting at data
-    size_t preferred_align; // preferred stride/start alignment for optimal copy
 };
 
 // Rendering API entrypoints. (Note: there are some additional hidden features

video/out/opengl/ra_gl.c

@@ -247,7 +247,6 @@ static struct ra_mapped_buffer *gl_create_mapped_buffer(struct ra *ra,
 
     struct ra_mapped_buffer *buf = talloc_zero(NULL, struct ra_mapped_buffer);
     buf->size = size;
-    buf->preferred_align = 1;
 
     struct ra_mapped_buffer_gl *buf_gl = buf->priv =
         talloc_zero(NULL, struct ra_mapped_buffer_gl);