Add WGSL support for the discard statement.

This is a trivial statement, mirroring GLSL exactly. It unlocks two
additional tests.

Change-Id: I17e47029cd53068bb832df6d119eac0af05114fd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/708796
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
5 files changed