Rename GrPipeline::Flags to GrPipeline::InputFlags

Renames the Flags to InputFlags and makes it a strongly typed enum.
Simplifies the relationship between GrSimpleMeshDrawOpHelper::Flags
and GrPipeline::InputFlags.

Bug: skia:
Change-Id: Ia25b8526ff96a47428dfbb82da18dbebdd382c5b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/204906
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>