Reland "For runtime shaders, deduce isOpaque automatically"

This is a reland of 995d16fc910219213755e924901c66b2a7c836a9

Original change's description:
> For runtime shaders, deduce isOpaque automatically
>
> The forceOpaque parameter is now ignored. Instead, we do a conservative
> analysis of the shader's main function to determine if it always returns
> an opaque color. This is good enough to detect simple cases, including
> things like:
>
>   return child.eval(p).rgb1
>
> Bug: skia:12643
> Bug: skia:12896
> Change-Id: I74b331aa12fadb1d0d1bb85f225dc7aa01ba2455
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/503346
> Reviewed-by: John Stiles <johnstiles@google.com>
> Commit-Queue: Brian Osman <brianosman@google.com>

Bug: skia:12643
Bug: skia:12896
Change-Id: I4ae55cdd3f88c1b5a08bd59913df1c1cd48a4679
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/503824
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Brian Osman <brianosman@google.com>
5 files changed