[mutter] clutter/offscreen-effect: Unscale first, then translate the FBO texture

[Marge Bot <marge-bot src gnome org>]

commit f887b02714f3b0d27409ed05a623790cd0340c32
Author: Daniel van Vugt <daniel van vugt canonical com>
Date:   Tue Dec 15 16:41:30 2020 +0800

    clutter/offscreen-effect: Unscale first, then translate the FBO texture
    
    We had been doing it backwards as far back as e3966882e8 which meant
    that we were translating by `fbo_offset / resource_scale` stage units
    instead of just `fbo_offset`.
    
    Because `fbo_offset` is in stage units already, it's not scaled and so
    needs to be applied after the unscaling from texels to stage units.
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1053>

 clutter/clutter/clutter-offscreen-effect.c | 21 ++++++++-------------
 1 file changed, 8 insertions(+), 13 deletions(-)
---
diff --git a/clutter/clutter/clutter-offscreen-effect.c b/clutter/clutter/clutter-offscreen-effect.c
index 42e5238c44..6f599670cb 100644
--- a/clutter/clutter/clutter-offscreen-effect.c
+++ b/clutter/clutter/clutter-offscreen-effect.c
@@ -436,19 +436,14 @@ clutter_offscreen_effect_paint_texture (ClutterOffscreenEffect *effect,
 {
   ClutterOffscreenEffectPrivate *priv = effect->priv;
   graphene_matrix_t transform;
-  float resource_scale;
-
-  graphene_matrix_init_translate (&transform,
-                                  &GRAPHENE_POINT3D_INIT (priv->fbo_offset_x,
-                                                          priv->fbo_offset_y,
-                                                          0.0f));
-
-  resource_scale = clutter_actor_get_resource_scale (priv->actor);
-  if (resource_scale != 1.0f)
-    {
-      float paint_scale = 1.0f / resource_scale;
-      graphene_matrix_scale (&transform, paint_scale, paint_scale, 1.f);
-    }
+  float unscale;
+
+  unscale = 1.0 / clutter_actor_get_resource_scale (priv->actor);
+  graphene_matrix_init_scale (&transform, unscale, unscale, 1.0);
+  graphene_matrix_translate (&transform,
+                             &GRAPHENE_POINT3D_INIT (priv->fbo_offset_x,
+                                                     priv->fbo_offset_y,
+                                                     0.0));
 
   if (!graphene_matrix_is_identity (&transform))
     {