Change SkSL::ShaderCaps and GrShaderCaps to structs.

There's no need for these fields to be private, and it removes the
need for friend classes -- the Caps classes can now set them directly.

Change-Id: I9b03d2ba06b792141036770f19804f100c05eeab
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/475258
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
17 files changed