Allow public SkSL to return vec4 (aka float4)

We already allowed narrowing type conversions, so GLSL type aliases
could be used almost exclusively. This fixes the last spot that a
client would be forced to use half4 rather than vec4.

Bug: skia:10679
Change-Id: Ie9cfc161650b238678861b9b126ce586c229162d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/332743
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
4 files changed