[gtk+/wip/paint-stack-cleanup: 2/4] gdkwindow: Only skip alpha blending if we cleared to the background

[Jasper St. Pierre <jstpierre src gnome org>]

commit fd898e01aafe6792d8d644a516780402c044c88f
Author: Jasper St. Pierre <jstpierre mecheye net>
Date:   Sat Jun 21 10:20:15 2014 -0400

    gdkwindow: Only skip alpha blending if we cleared to the background
    
    Currently, this always happens, but some consumers of begin_paint_region
    want to be able to start a paint that doesn't end up clearing to the
    background pattern. We'll expose a new API for this and remove the
    automatic clear from begin_paint_region.

 gdk/gdkinternals.h |    1 +
 gdk/gdkwindow.c    |   10 ++++++++--
 2 files changed, 9 insertions(+), 2 deletions(-)
---
diff --git a/gdk/gdkinternals.h b/gdk/gdkinternals.h
index 32754a3..331e6d9 100644
--- a/gdk/gdkinternals.h
+++ b/gdk/gdkinternals.h
@@ -209,6 +209,7 @@ struct _GdkWindow
     cairo_region_t *region;
     cairo_surface_t *surface;
     gboolean surface_needs_composite;
+    gboolean cleared_to_background;
   } current_paint;
 
   cairo_region_t *update_area;
diff --git a/gdk/gdkwindow.c b/gdk/gdkwindow.c
index ad93c9b..b43feeb 100644
--- a/gdk/gdkwindow.c
+++ b/gdk/gdkwindow.c
@@ -1826,6 +1826,7 @@ gdk_window_free_current_paint (GdkWindow *window)
   window->current_paint.region = NULL;
 
   window->current_paint.surface_needs_composite = FALSE;
+  window->current_paint.cleared_to_background = FALSE;
 }
 
 /**
@@ -2832,9 +2833,10 @@ gdk_window_end_paint (GdkWindow *window)
       cairo_clip (cr);
 
       /* We can skip alpha blending for a fast composite case
-       * if we have an impl window or we're a fully opaque window. */
+       * if we have an impl window or we're a fully opaque window that
+       * has been cleared to the background pattern. */
       skip_alpha_blending = (gdk_window_has_impl (window) ||
-                             window->alpha == 255);
+                             (window->alpha == 255 && window->current_paint.cleared_to_background));
 
       if (skip_alpha_blending)
         {
@@ -2976,6 +2978,10 @@ gdk_window_clear_backing_region (GdkWindow *window)
   cairo_fill (cr);
 
   cairo_destroy (cr);
+
+  /* Since we've cleared to an opaque pattern, we can skip alpha
+   * blending later on in the composite stage. */
+  window->current_paint.cleared_to_background = TRUE;
 }
 
 /* This returns either the current working surface on the paint stack