Don't define SK_GL when no other GPU backends are enabled

SK_GL must explicitly be defined to enable the GL backend,
e.g. by setting skia_use_gl in gn args.

Change-Id: I61008b5422f4690e6f012609b239cd74e63d9f92
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/306732
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed