[mutter] screen-cast/area-src: Clear framebuffer before painting stage

[Florian Müllner <fmuellner src gnome org>]

commit df228e8945cb77e517f89af69bc1fba4eef89279
Author: Jonas Ådahl <jadahl gmail com>
Date:   Wed Sep 30 17:40:10 2020 +0200

    screen-cast/area-src: Clear framebuffer before painting stage
    
    We'll be painting to a framebuffer that may not be completely covered by
    the painted areas, meaning the not painted areas would end up undefined,
    thus potentially contain garbage or old content.
    
    Avoid this by clearing the framebuffer before painting the stage.
    
    Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1442
    
    https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1459

 clutter/clutter/clutter-paint-context.h         | 1 +
 clutter/clutter/clutter-stage.c                 | 8 ++++++++
 src/backends/meta-screen-cast-area-stream-src.c | 4 ++--
 3 files changed, 11 insertions(+), 2 deletions(-)
---
diff --git a/clutter/clutter/clutter-paint-context.h b/clutter/clutter/clutter-paint-context.h
index 8461b947ac..2a36fe9679 100644
--- a/clutter/clutter/clutter-paint-context.h
+++ b/clutter/clutter/clutter-paint-context.h
@@ -34,6 +34,7 @@ typedef enum _ClutterPaintFlag
   CLUTTER_PAINT_FLAG_NONE = 0,
   CLUTTER_PAINT_FLAG_NO_CURSORS = 1 << 0,
   CLUTTER_PAINT_FLAG_FORCE_CURSORS = 1 << 1,
+  CLUTTER_PAINT_FLAG_CLEAR = 1 << 2,
 } ClutterPaintFlag;
 
 #define CLUTTER_TYPE_PAINT_CONTEXT (clutter_paint_context_get_type ())
diff --git a/clutter/clutter/clutter-stage.c b/clutter/clutter/clutter-stage.c
index 14e04e1a5c..f5d7f141b4 100644
--- a/clutter/clutter/clutter-stage.c
+++ b/clutter/clutter/clutter-stage.c
@@ -3614,6 +3614,14 @@ clutter_stage_paint_to_framebuffer (ClutterStage                *stage,
   ClutterPaintContext *paint_context;
   cairo_region_t *redraw_clip;
 
+  if (paint_flags & CLUTTER_PAINT_FLAG_CLEAR)
+    {
+      CoglColor clear_color;
+
+      cogl_color_init_from_4ub (&clear_color, 0, 0, 0, 0);
+      cogl_framebuffer_clear (framebuffer, COGL_BUFFER_BIT_COLOR, &clear_color);
+    }
+
   redraw_clip = cairo_region_create_rectangle (rect);
   paint_context =
     clutter_paint_context_new_for_framebuffer (framebuffer,
diff --git a/src/backends/meta-screen-cast-area-stream-src.c b/src/backends/meta-screen-cast-area-stream-src.c
index e22b7dcabf..e7d42d432a 100644
--- a/src/backends/meta-screen-cast-area-stream-src.c
+++ b/src/backends/meta-screen-cast-area-stream-src.c
@@ -414,7 +414,7 @@ meta_screen_cast_area_stream_src_record_to_buffer (MetaScreenCastStreamSrc  *src
   MetaRectangle *area;
   float scale;
   int stride;
-  ClutterPaintFlag paint_flags = CLUTTER_PAINT_FLAG_NONE;
+  ClutterPaintFlag paint_flags = CLUTTER_PAINT_FLAG_CLEAR;
 
   stage = get_stage (area_src);
   area = meta_screen_cast_area_stream_get_area (area_stream);
@@ -456,7 +456,7 @@ meta_screen_cast_area_stream_src_record_to_framebuffer (MetaScreenCastStreamSrc
   ClutterStage *stage;
   MetaRectangle *area;
   float scale;
-  ClutterPaintFlag paint_flags = CLUTTER_PAINT_FLAG_NONE;
+  ClutterPaintFlag paint_flags = CLUTTER_PAINT_FLAG_CLEAR;
 
   stage = CLUTTER_STAGE (meta_backend_get_stage (backend));
   area = meta_screen_cast_area_stream_get_area (area_stream);