cut SK_SAMPLES_FOR_X, simplify SK_?32_SHIFT

SK_SAMPLES_FOR_X doesn't do anything other than set the SK_?32_SHIFT
macros, so we can just set them instead, and further we can derive
everything we need from SK_R32_SHIFT.

Change-Id: I766d00c868da04939208353d09201f189a99c0c7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/210121
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Hal Canary <halcanary@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
5 files changed