Minor cleanup in the sample_matrix GMs

While trying to debug matrix sampling, I wanted to make these
easier to work with.

Change-Id: I24889277e7bf0dbdabf012028248c32641c98232
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/292838
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed