Add support for mesh programs to sksl-minify.

Mesh programs use flags --meshflag and --meshvert.

The input programs must specify Varyings and Attribute structs that
match the SkMeshSpecification; the minified output will eliminate
these structs for convenience.

Change-Id: Ic0b3efc4c6d9fcd9684d7ff2d9f21448af09db75
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/755939
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Chris Mumford <cmumford@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
14 files changed