Bug 783368 - Add a pref to control low precision tile rendering. r=bgirard

Add pref 'layers.low-precision-buffer', enabled on mobile/android.

gfx/layers/basic/BasicTiledThebesLayer.cpp

@@ -591,7 +591,8 @@ BasicTiledThebesLayer::PaintThebes(gfxContext* aContext,
   // If we have a critical display-port defined, render the full display-port
   // progressively in the low-precision tiled buffer.
   bool updatedLowPrecision = false;
-  if (!criticalDisplayPort.IsEmpty() &&
+  if (gfxPlatform::UseLowPrecisionBuffer() &&
+      !criticalDisplayPort.IsEmpty() &&
       !nsIntRegion(layerDisplayPort).Contains(mVisibleRegion)) {
     nsIntRegion oldValidRegion = mLowPrecisionTiledBuffer.GetValidRegion();
     oldValidRegion.And(oldValidRegion, mVisibleRegion);

gfx/thebes/gfxPlatform.cpp

@@ -1246,6 +1246,22 @@ gfxPlatform::UseProgressiveTilePainting()
     return sUseProgressiveTilePainting;
 }
 
+bool
+gfxPlatform::UseLowPrecisionBuffer()
+{
+    static bool sUseLowPrecisionBuffer;
+    static bool sUseLowPrecisionBufferPrefCached = false;
+
+    if (!sUseLowPrecisionBufferPrefCached) {
+        sUseLowPrecisionBufferPrefCached = true;
+        mozilla::Preferences::AddBoolVarCache(&sUseLowPrecisionBuffer,
+                                              "layers.low-precision-buffer",
+                                              false);
+    }
+
+    return sUseLowPrecisionBuffer;
+}
+
 bool
 gfxPlatform::UseReusableTileStore()
 {

gfx/thebes/gfxPlatform.h

@@ -405,6 +405,10 @@ public:
     // Break large OMTC tiled thebes layer painting into small paints.
     static bool UseProgressiveTilePainting();
 
+    // When a critical display-port is set, render the visible area outside of
+    // it into a buffer at a lower precision. Requires tiled buffers.
+    static bool UseLowPrecisionBuffer();
+
     // Retain some invalid tiles when the valid region of a layer changes and
     // excludes previously valid tiles.
     static bool UseReusableTileStore();

mobile/android/app/mobile.js

@@ -523,6 +523,7 @@ pref("layers.acceleration.disabled", false);
 pref("layers.offmainthreadcomposition.enabled", true);
 pref("layers.async-video.enabled", true);
 pref("layers.progressive-paint", true);
+pref("layers.low-precision-buffer", true);
 
 pref("notification.feature.enabled", true);