Fix fuzzer-discovered error with child calls.

The `eval` methods take a shader/blender/colorFilter, and we assumed
when assembling the ChildCall expression that the child expression would
be a VariableReference because opaque objects don't participate in
normal expressions. However, comma-expressions were allowed to contain
opaque types. GLSL doesn't allow opaque types in comma-expressions:

http://screen/8YW59tYDUbBh9eW

Now we disallow them as well.

Change-Id: Iaf88ef7bddb5cc8f1f1e23b515174dfc291e00c7
Bug: oss-fuzz:41072
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/472446
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
4 files changed