Initialize font texture cache.

Now that text is renderer using bilinear filtering, each character is surrounded by
a 1 pixel border. This border needs to be initialized to a transparent value.

Change-Id: I26d3195db4bd576ac1f2da4c8f8e4dd18768bf12
diff --git a/libs/hwui/FontRenderer.cpp b/libs/hwui/FontRenderer.cpp
index 4e4a277..5485113 100644
--- a/libs/hwui/FontRenderer.cpp
+++ b/libs/hwui/FontRenderer.cpp
@@ -426,6 +426,8 @@
 
 void FontRenderer::initTextTexture() {
     mTextTexture = new uint8_t[mCacheWidth * mCacheHeight];
+    memset(mTextTexture, 0, mCacheWidth * mCacheHeight * sizeof(uint8_t));
+
     mUploadTexture = false;
 
     glGenTextures(1, &mTextureId);