make set3DMask virtual, so we can safely notify the shadercontext

also boost preallocated storage to account for this combo of bitmapshader + emboss + colorfilter

BUG=skia:
R=djsollen@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/563563002

bug:16307475
Change-Id: I404fa776511a27b7be1b3b41dcb06b8b828a4716
6 files changed