Add runtime-shader entrypoint to WGSL code generator.

Previously, attempting to compile a runtime shader would emit an
error that the program-kind is unsupported; now, we synthesize
a basic entrypoint which just calls straight through to main.

This will only be used by skslc, in order to let us use .rts files
as WGSL compilation test cases. The entrypoint is only good enough
to pass the WGSL validator; the binding locations aren't meaningful.
Skia won't ever compile runtime shaders to WGSL directly.

Change-Id: Ic5e3f2eda1c501e4ebb6f2811ab1cbd7ee059584
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/705617
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Arman Uguray <armansito@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Arman Uguray <armansito@google.com>
6 files changed