[graphite] Add geometry uniforms to RenderStep

RenderSteps expose the uniforms they use for their portion of the
program, and assume will be bound independently of the other uniform
sets needed for the paint, etc.

This wires them up in DrawPass, although the RenderSteps currently don't
define any. At the moment this CL is really just laying out the API and
then it will be tested more thoroughly in a follow up when
CommandBufferTest moves its hard-coded test shaders into the RenderStep

Bug: skia:12466
Change-Id: I8287dadb9a458127ba9f376ba351ca1932788c71
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/474137
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
3 files changed