Change SkSL main() from (float x, float y) to (float2 p)

Change-Id: Id046199edd63535ef07e1dfa65fbc7c0f8cefd00
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/269371
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
14 files changed