Consistently fail readPixels when rowbytes not a multiple of bpp

Previously this would report success but may have rounded the
rowbytes down to a multiple of bpp prior to writing the dst.
On GPU it could trigger an assert in a debug build.

Bug: chromium:1163061
Change-Id: I19709f4cdb71139732998a4dd2e14476099f0ba8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/363782
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Mike Reed <reed@google.com>
10 files changed