Add support in Metal for builtin functions using builtin globals.
Add support in Metal for builtin functions using builtin globals.

The Metal function-requirements checking logic assumed that a builtin
function would never have any requirements. This would cause us to
generate invalid Metal code if a builtin function included a reference
to sk_FragCoord.

Change-Id: I9992981b03306b254a3fc4b87b940ddb4c646bf1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/525182
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Reviewed-by: Ethan Nicholas <d2cfb8c2f603266f4dc3a6f09acbcc0089603b37@google.com>
Commit-Queue: Ethan Nicholas <d2cfb8c2f603266f4dc3a6f09acbcc0089603b37@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia