Add ES3 intrinsics isinf/isnan to public SkSL ES3.

The ES3 spec doesn't mandate that `isnan` actually has to do anything,
so the Isnan test is not enabled. (It doesn't work on my personal
machine unless I make the NaN detectable at compile-time.)

We do not support these functions in constant-expressions, as we
currently avoid optimizing anything into a non-finite value; we leave
expressions alone if we calculate a NaN/inf result for their value.

Change-Id: Ibfdfb47b6e6134165c8780db570de04a916d2bfa
Bug: skia:12022
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/441581
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
11 files changed