Add shader-based text gamma correction

To enable it, the system property ro.hwui.text_gamma_shader must be
set to true. For testing, DEBUG_FONT_RENDERER_FORCE_SHADER_GAMMA
can be set to 1 in libhwui/Debug.h.

Change-Id: If345c6b71b67ecf1ef2e8847b71f30f3ef251a27
diff --git a/libs/hwui/Debug.h b/libs/hwui/Debug.h
index 55a860e..13780c3 100644
--- a/libs/hwui/Debug.h
+++ b/libs/hwui/Debug.h
@@ -65,6 +65,9 @@
 // Turn on to enable additional debugging in the font renderers
 #define DEBUG_FONT_RENDERER 0
 
+// Force gamma correction in shaders
+#define DEBUG_FONT_RENDERER_FORCE_SHADER_GAMMA 0
+
 // Turn on to dump display list state
 #define DEBUG_DISPLAY_LIST 0