Change samplerVariable() to return a const char *.

This allows backends to use an expression rather than a variable for
sampler access.

Change-Id: I2346ac418c8d6277416a4430e2eaf6ab3ffee0a2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/222036
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
14 files changed