make legacy shadercontext optional

Current savings : 96K on intel mac

Breaks -- these subclass don't have a pipeline
- emboss
- perlinnoise

For now, shadercontext is still on -- clients must opt-out with SK_DISABLE_LEGACY_SHADERCONTEXT

Bug: skia:
Change-Id: Idd14cae4013e8de92f16b0707f8b8d43bb6d11dd
Reviewed-on: https://skia-review.googlesource.com/c/162362
Auto-Submit: Mike Reed <reed@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
18 files changed