Report an error if sk_LastFragColor is referenced without fbFetchSupport

Adjusted default caps in skslc to be consistent with runtime behavior,
and added optional settings mode to enable the feature. Tests for both
scenarios. (The error test crashed prior to the fix).

Bug: oss-fuzz:38726
Change-Id: I5270d4837ac982085d7baf5abd4b361f7bfb8562
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/449062
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
11 files changed