Make SkGpuDevice::drawVertices perform color byte order swap and premul step using...

Make SkGpuDevice::drawVertices perform color byte order swap and premul step using vertex shader.

Change-Id: I8153ba8c6bb48d8b15d524fbfafbe3c6d83f39c5
Reviewed-on: https://skia-review.googlesource.com/7727
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>