Fix the GLSL persistent cache

Store (up to) three GLSL strings in the cache entry, along with the
bookkeeping top reconstruct them.

This is a Cherry-Pick (minus testing changes that don't merge) of:
  https://skia-review.googlesource.com/c/skia/+/205823

No-Tree-Checks: true
No-Try: true
No-Presubmit: true
Change-Id: I93dca43e7e7395e54c1c71abaf0af7576734526b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/205920
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed