New read pixels implementation that is simpler but does all conversions on CPU.

Change-Id: Ia548cd24a8544b35a233311706faf48de353b7cf
Reviewed-on: https://skia-review.googlesource.com/109902
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
18 files changed