b=1015218 add pref to remove Xlib offscreen surfaces on GTK3 platform r=karlt

This patch adds a new pref called "layers.use-image-offscreen-surfaces"
that makes GTK3 platform create gfxImageSurfaces for content layers
instead of gfxXlibSurfaces.

--HG--
extra : rebase_source : b803a453bafc70226f3d4c65684618f154147fdb

gfx/thebes/gfxPlatformGtk.cpp

@@ -93,7 +93,9 @@ gfxPlatformGtk::CreateOffscreenSurface(const IntSize& size,
     // we should try to match
     GdkScreen *gdkScreen = gdk_screen_get_default();
     if (gdkScreen) {
-        if (UseXRender()) {
+        // When forcing Thebes Layers to use image surfaces for content,
+        // force creation of gfxImageSurface surfaces.
+        if (UseXRender() && !UseImageOffscreenSurfaces()) {
             Screen *screen = gdk_x11_screen_get_xscreen(gdkScreen);
             XRenderPictFormat* xrenderFormat =
                 gfxXlibSurface::FindRenderFormat(DisplayOfScreen(screen),

gfx/thebes/gfxPlatformGtk.h

@@ -7,6 +7,7 @@
 #define GFX_PLATFORM_GTK_H
 
 #include "gfxPlatform.h"
+#include "gfxPrefs.h"
 #include "nsAutoRef.h"
 #include "nsTArray.h"
 
@@ -88,6 +89,16 @@ public:
 #endif
     }
 
+    bool UseImageOffscreenSurfaces() {
+        // We want to turn on image offscreen surfaces ONLY for GTK3 builds
+        // since GTK2 theme rendering still requires xlib surfaces per se.
+#if (MOZ_WIDGET_GTK == 3)
+        return gfxPrefs::UseImageOffscreenSurfaces();
+#else
+        return false;
+#endif
+    }
+
     virtual gfxImageFormat GetOffscreenFormat();
 
     virtual int GetScreenDepth() const;

gfx/thebes/gfxPrefs.h

@@ -240,6 +240,7 @@ private:
   DECL_GFX_PREF(Live, "layers.offmainthreadcomposition.frame-rate", LayersCompositionFrameRate, int32_t,-1);
   DECL_GFX_PREF(Once, "layers.offmainthreadcomposition.force-enabled", LayersOffMainThreadCompositionForceEnabled, bool, false);
   DECL_GFX_PREF(Once, "layers.offmainthreadcomposition.testing.enabled", LayersOffMainThreadCompositionTestingEnabled, bool, false);
+  DECL_GFX_PREF(Once, "layers.use-image-offscreen-surfaces",   UseImageOffscreenSurfaces, bool, false);
   DECL_GFX_PREF(Live, "layers.orientation.sync.timeout",       OrientationSyncMillis, uint32_t, (uint32_t)0);
   DECL_GFX_PREF(Once, "layers.prefer-d3d9",                    LayersPreferD3D9, bool, false);
   DECL_GFX_PREF(Once, "layers.prefer-opengl",                  LayersPreferOpenGL, bool, false);

widget/shared/nsShmImage.cpp

@@ -12,7 +12,9 @@
 #endif
 
 #include "nsShmImage.h"
-#include "gfxPlatform.h"
+#ifdef MOZ_WIDGET_GTK
+#include "gfxPlatformGtk.h"
+#endif
 #include "gfxImageSurface.h"
 
 #ifdef MOZ_HAVE_SHMIMAGE
@@ -24,9 +26,11 @@ using namespace mozilla::ipc;
 static bool gShmAvailable = true;
 bool nsShmImage::UseShm()
 {
-    return gfxPlatform::GetPlatform()->
-        ScreenReferenceSurface()->GetType() == gfxSurfaceType::Image
-        && gShmAvailable;
+#ifdef MOZ_WIDGET_GTK
+    return (gShmAvailable && !gfxPlatformGtk::GetPlatform()->UseXRender());
+#else
+    return gShmAvailable;
+#endif
 }
 
 already_AddRefed<nsShmImage>