Update SkSLTest to check a 2x2 surface.

Derivative tests don't really make intuitive sense in a 1x1 surface.
This should give the derivatives a better workout.

Change-Id: I09e5ff2b7815d42d1874e5deef360e9bc2e83aff
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/449195
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
1 file changed