In pipeline stage generator, never emit declarations for opaque types

This only affects fragmentProcessors (children) - and the backend SkSL
we're emitting should not contain those. We've just been silently
ignoring those declarations when converting to GLSL, MSL, etc.

Change-Id: I241f2f4fe4614b49ebccc9c2976fd408e94656d0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/384316
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
6 files changed