[graphite] Shorten shader names w/o relying on auto-trimming

This is motivated by:
 https://skia-review.googlesource.com/c/skia/+/955976
in which we don't want to automatically trim user-provided names.

Change-Id: I2f28291ab6be5e025b625d490586f8890db31d59
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/956025
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed