Add golden outputs for the Metal backend.

This CL also updates the blend tests to use sk_FragColor instead of
returning a half4, as the Metal backend assumes that a fragment
processor's `main` will return void and always synthesizes a
`return *_out;` at the end.

(context link:
https://osscs.corp.google.com/android/platform/superproject/+/master:external/skqp/src/sksl/SkSLMetalCodeGenerator.cpp;l=803;drc=842d31b14159626054e01dd32826563a8f4214bf )

BYPASS_INCLUSIVE_LANGUAGE_REASON=see http://b/168134166

Change-Id: I330a456bf25ee72d3a29c59cd624625378ae80a0
Bug: skia:10649, skia:10757, skia:10758, skia:10759, skia:10760, skia:10761, skia:10762
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/319409
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
219 files changed