Fix SkTextBlob offset semantics.

Implement proper x/y drawTextBlob() handling by plumbing a
drawPosText() offset parameter (to act as an additional glyph pos
translation) throughout the device layer.

The new offset superceeds the existing constY, with a minor semantic
tweak: whereas previous implementations were ignoring constY in 2D
positioning mode (scalarsPerGlyph == 2), now the offset is always
observed, in all positioning modes. We can do this because existing
drawPosText() clients always pass constY == 0 for full positioning mode.

R=reed@google.com, jvanverth@google.com, robertphillips@google.com

Review URL: https://codereview.chromium.org/605533002
diff --git a/src/utils/SkDeferredCanvas.cpp b/src/utils/SkDeferredCanvas.cpp
index c8402dd..06f7bb0 100644
--- a/src/utils/SkDeferredCanvas.cpp
+++ b/src/utils/SkDeferredCanvas.cpp
@@ -210,8 +210,8 @@
                             SkScalar x, SkScalar y, const SkPaint& paint) SK_OVERRIDE
         {SkASSERT(0);}
     virtual void drawPosText(const SkDraw&, const void* text, size_t len,
-                                const SkScalar pos[], SkScalar constY,
-                                int scalarsPerPos, const SkPaint& paint) SK_OVERRIDE
+                             const SkScalar pos[], int scalarsPerPos,
+                             const SkPoint& offset, const SkPaint& paint) SK_OVERRIDE
         {SkASSERT(0);}
     virtual void drawTextOnPath(const SkDraw&, const void* text,
                                 size_t len, const SkPath& path,