Fix the GLSL persistent cache (and add a config to test it)

We need to store (up to) three GLSL strings in the cache entry,
along with the bookkeeping to reconstruct them. To make things
simpler, we now store the null terminators.

Change-Id: Ic4fe03cb5d774464372ceec8740da1bfe9069550
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/205823
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
12 files changed