[sksl] Add SK_POINTSIZE_BUILTIN constant

GLSL, Metal, and WGSL code generators now determine the sk_PointSize
builtin by modifier value instead of by name. Also updated GLSL code
generator to use SK_POSITION_BUILTIN when processing sk_Position.

Change-Id: I57418f66786dd2bac8c4a7d8c34f83221daf505c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/541719
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Arman Uguray <armansito@google.com>
4 files changed