add SkRect::joinNonEmptyArg for faster unioning

src/gpu/GrBitmapTextContext.cpp

Index: src/gpu/GrBitmapTextContext.cpp
diff --git a/src/gpu/GrBitmapTextContext.cpp b/src/gpu/GrBitmapTextContext.cpp
index 422a7e0179b2242b89f340eadc6e441e04dfb1e0..c9cdf2c23f926430d1d06854808cd095d64c287e 100755
--- a/src/gpu/GrBitmapTextContext.cpp
+++ b/src/gpu/GrBitmapTextContext.cpp
@@ -575,7 +575,7 @@ HAS_ATLAS:
     r.fRight = SkFixedToFloat(vx + width);
     r.fBottom = SkFixedToFloat(vy + height);
 
-    fVertexBounds.growToInclude(r);
+    fVertexBounds.joinNonEmptyArg(r);
 
     size_t vertSize = useColorVerts ? (2 * sizeof(SkPoint) + sizeof(GrColor)) :
                                       (2 * sizeof(SkPoint));