more easy lowp shader stages

This fills out a couple more matrix and gather stages.

Deletes a not particularly important unit test that was using a
scale matrix in a weird, non-lowp compatible way.

This will require guards for Blink layout tests.

Change-Id: I54cb228ff541f771e8f4758f07d26c5161d48af3
Reviewed-on: https://skia-review.googlesource.com/62520
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
5 files changed