New private API for "deferred" runtime shaders

Allows generating uniforms (via callback) as late as possible.

Change-Id: I2003922c14b5968ae7ab675f6ba57b3f1679fb7e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/653716
Reviewed-by: John Reck <jreck@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
3 files changed