Add extended option [wgsl=true] to enable WGSL in DM.

You can now use `dm --config "graphite[api=dawn_mtl,wgsl=true]"`
to run the dm test slides with the WGSL code generator instead of
SPIR-V.

Bug: b/40044196
Change-Id: I07f64542ec35fc75a5785bb0ea0efc2a248614a2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/745062
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
5 files changed