Move the rt adjust uniform into GP EmitArgs

Move the rt adjust uniform into GP EmitArgs

The GP will likely require this value when dealing with a geometry
shader. In the future we may wish to either switch to device-space
geometry shaders, or else put this value in an "sk_" builtin.

BUG=skia:

Change-Id: I8dff88fc219feef84d39fb7bbd08f3b5686f53d2
Reviewed-on: https://skia-review.googlesource.com/8362
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>