Implement compile-time optimization for refract().

$genType refract($genType I, $genType N, float eta);
$genHType refract($genHType I, $genHType N, half eta);

The half form of refract was originally taking a `float eta` in our
headers, which seems wrong (and causes the DSL to break unless you add
casts). I've corrected the headers to use `half eta`.

Change-Id: I74b9ac330e0f7e99622d19cf7365aaa4cc910e57
Bug: skia:12034
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/412664
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
9 files changed