Add ES3 intrinsic modf to sksl_public.

The test has been improved and now covers a variety of values. Because
this intrinsic has side-effects (an out-param), we do not support
optimizing it or treating it as a constant-expression.

The modf documentation doesn't mention anything about constant-
expression support or lack thereof. Experimentally, modf is also not
treated as a constant-expression by Apple GLSL or glslang:

http://screen/4RWwYKr6vCjxCPQ
http://screen/45ttDTVAFGDRyxP

Change-Id: I15bb1de80e90fa97ddf8e9d3803352603b9608d0
Bug: skia:12202
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/446396
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
7 files changed