Reduce stencil buffer clearing

TBR=bsalomon@google.com
Bug: skia:6953
Change-Id: I079f90711297ee290f2d4011cfcb18b764554deb
Reviewed-on: https://skia-review.googlesource.com/40691
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
10 files changed