Explicit tile bounds for SkPictureShader

The integer picture size is not granular enough to allow precise tiling
in arbitrary coordinate systems. This CL adds an optional tile bounds
float rect param to control the tile size and location.

(this also allows tile spacing emulation for picture
shaders).

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

Author: fmalita@chromium.org

Review URL: https://codereview.chromium.org/437393003
8 files changed