Bug 831438. Add pref for forcing 16bit on B2G. r=cjones

B2G currently tries to detect the depth of the screen and uses
that as the offscreen format. This lets us override that
for testing the 16bit path on 24bit devices.

gfx/thebes/gfxAndroidPlatform.cpp

@@ -7,6 +7,7 @@
 
 #include "gfxAndroidPlatform.h"
 #include "mozilla/gfx/2D.h"
+#include "mozilla/Preferences.h"
 
 #include "gfxFT2FontList.h"
 #include "gfxImageSurface.h"
@@ -102,6 +103,11 @@ gfxAndroidPlatform::gfxAndroidPlatform()
     mOffscreenFormat = mScreenDepth == 16
                        ? gfxASurface::ImageFormatRGB16_565
                        : gfxASurface::ImageFormatRGB24;
+
+    if (Preferences::GetBool("gfx.android.rgb16.force", false)) {
+        mOffscreenFormat = gfxASurface::ImageFormatRGB16_565;
+    }
+
 }
 
 gfxAndroidPlatform::~gfxAndroidPlatform()