Make GL_RED workaround only be triggered by osmesa

BUG=skia:6134

Change-Id: If9c8fb55973815d378cd8a9d956735656d4c097c
Reviewed-on: https://skia-review.googlesource.com/7276
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed