passing fragcoord.w in SPIR-V and Metal

Bug: skia:
Change-Id: I94447ae74a1709c789b8d70dfd7f9572bbb2c3cf
Reviewed-on: https://skia-review.googlesource.com/c/174303
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
2 files changed