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/gpu/GrTextContext.h b/src/gpu/GrTextContext.h
index c139e22..da41c1b 100644
--- a/src/gpu/GrTextContext.h
+++ b/src/gpu/GrTextContext.h
@@ -28,8 +28,8 @@
                           SkScalar x, SkScalar y) = 0;
     virtual void drawPosText(const GrPaint&, const SkPaint&,
                              const char text[], size_t byteLength,
-                             const SkScalar pos[], SkScalar constY,
-                             int scalarsPerPosition) = 0;
+                             const SkScalar pos[], int scalarsPerPosition,
+                             const SkPoint& offset) = 0;
 
     virtual bool canDraw(const SkPaint& paint) = 0;