This basically wraps up the old `uniforms` and `buf` params
into a new type that has push() and pushF() methods that return
a value you can pass directly to Builder::uniform32() and co.
I think this has uniforms about as streamlined as they can get.
Change-Id: I8f611f91b4a2d7cdb8f05ce0333669d2e3930be8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/252937
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1cc6067757