[mutter] backends: Don't include damage history in swap region

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

commit a82dd68605f4c0f7c9d173ba2f34d7cb57fc8c47
Author: Erico Nunes <nunes erico gmail com>
Date:   Mon Nov 15 12:48:05 2021 +0100

    backends: Don't include damage history in swap region
    
    Following the EGL_KHR_swap_buffers_with_damage specification, the
    surface damage used by eglSwapBuffersWithDamage does not need to
    contain the damage history.
    Rework that to reduce the amount of rectangles that get passed to
    the backend.
    
    Also rework some of the regions that were using fb_clip_region and
    missing the last scaling to support fractional scaling.
    
    Signed-off-by: Erico Nunes <nunes erico gmail com>
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2089>

 src/backends/meta-stage-impl.c | 49 ++++++++++++++++++++++++++----------------
 1 file changed, 30 insertions(+), 19 deletions(-)
---
diff --git a/src/backends/meta-stage-impl.c b/src/backends/meta-stage-impl.c
index 2990c89779..d41114b625 100644
--- a/src/backends/meta-stage-impl.c
+++ b/src/backends/meta-stage-impl.c
@@ -445,6 +445,8 @@ meta_stage_impl_redraw_view_primary (MetaStageImpl    *stage_impl,
   gboolean has_buffer_age;
   gboolean swap_with_damage;
   cairo_region_t *redraw_clip;
+  cairo_region_t *redraw_clip_history;
+  cairo_region_t *redraw_clip_swap;
   cairo_region_t *queued_redraw_clip = NULL;
   cairo_region_t *fb_clip_region;
   cairo_region_t *swap_region;
@@ -567,21 +569,29 @@ meta_stage_impl_redraw_view_primary (MetaStageImpl    *stage_impl,
 
   if (use_clipped_redraw)
     {
-      /* Regenerate redraw_clip because:
-       *  1. It's missing the regions added from damage_history above; and
-       *  2. If using fractional scaling then it might be a fraction of a
-       *     logical pixel (or one physical pixel) smaller than
-       *     fb_clip_region, due to the clamping from
-       *     offset_scale_and_clamp_region. So we need to ensure redraw_clip
-       *     is a superset of fb_clip_region to avoid such gaps.
+      /* If using fractional scaling then the regions might be a fraction of
+       * a logical pixel (or one physical pixel) smaller than expected.
+       * So we need to regenerate them to avoid such gaps.
        */
-      cairo_region_destroy (redraw_clip);
-      redraw_clip = scale_offset_and_clamp_region (fb_clip_region,
-                                                   1.0 / fb_scale,
-                                                   view_rect.x,
-                                                   view_rect.y);
+      redraw_clip_history = scale_offset_and_clamp_region (fb_clip_region,
+                                                           1.0 / fb_scale,
+                                                           view_rect.x,
+                                                           view_rect.y);
+
+      redraw_clip_swap = scale_offset_and_clamp_region (redraw_clip,
+                                                        1.0 / fb_scale,
+                                                        view_rect.x,
+                                                        view_rect.y);
+    }
+  else
+    {
+      redraw_clip_history = cairo_region_reference (fb_clip_region);
+      redraw_clip_swap = cairo_region_reference (redraw_clip);
     }
 
+  g_clear_pointer (&redraw_clip, cairo_region_destroy);
+  g_clear_pointer (&fb_clip_region, cairo_region_destroy);
+
   if (paint_debug_flags & CLUTTER_DEBUG_PAINT_DAMAGE_REGION)
     {
       cairo_region_t *debug_redraw_clip;
@@ -592,11 +602,11 @@ meta_stage_impl_redraw_view_primary (MetaStageImpl    *stage_impl,
     }
   else if (use_clipped_redraw)
     {
-      queue_damage_region (stage_window, stage_view, fb_clip_region);
+      queue_damage_region (stage_window, stage_view, redraw_clip_history);
 
-      cogl_framebuffer_push_region_clip (fb, fb_clip_region);
+      cogl_framebuffer_push_region_clip (fb, redraw_clip_history);
 
-      paint_stage (stage_impl, stage_view, redraw_clip);
+      paint_stage (stage_impl, stage_view, redraw_clip_history);
 
       cogl_framebuffer_pop_clip (fb);
     }
@@ -604,9 +614,11 @@ meta_stage_impl_redraw_view_primary (MetaStageImpl    *stage_impl,
     {
       meta_topic (META_DEBUG_BACKEND, "Unclipped stage paint");
 
-      paint_stage (stage_impl, stage_view, redraw_clip);
+      paint_stage (stage_impl, stage_view, redraw_clip_history);
     }
 
+  cairo_region_destroy (redraw_clip_history);
+
   /* XXX: It seems there will be a race here in that the stage
    * window may be resized before the cogl_onscreen_swap_region
    * is handled and so we may copy the wrong region. I can't
@@ -616,12 +628,11 @@ meta_stage_impl_redraw_view_primary (MetaStageImpl    *stage_impl,
    * artefacts.
    */
   if (use_clipped_redraw)
-    swap_region = cairo_region_reference (fb_clip_region);
+    swap_region = cairo_region_reference (redraw_clip_swap);
   else
     swap_region = cairo_region_create ();
 
-  g_clear_pointer (&redraw_clip, cairo_region_destroy);
-  g_clear_pointer (&fb_clip_region, cairo_region_destroy);
+  cairo_region_destroy (redraw_clip_swap);
 
   COGL_TRACE_BEGIN_SCOPED (MetaStageImplRedrawViewSwapFramebuffer,
                            "Paint (swap framebuffer)");