clean read/write pixels signatures, augment SkSurface API

For now, not adding writePixels to surface, since it appears we may not
be able to remove writePixels from canvas :(

Bug: skia:3216
Change-Id: I64ccebb31effacffe615ae4537fb46a161a6768d
Reviewed-on: https://skia-review.googlesource.com/25562
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Reed <reed@google.com>
11 files changed