add array to the Uniforms

Note: I have changed from using a byte offset for index to
an ordinary array index at the builder interface, but the
builder converts this to a byte offset for the instruction.
This makes the API easier to use.

I've added pushArray, and pushArrayF to the Uniforms, and
convenience methods on the builder to take Uniforms.

I've expanded the tests to use the new API.

Change-Id: Id538e826a96d4d242ae6482acc711d84c9041239
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/432036
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed