Remove use of texture swizzle in GL backend.

This has always been a potential source of a bug. If the same texture is
used twice in a shader with different swizzles we would overwrite the
swizzle for the first use by that of the second use since there is
only one fixed function swizzle per texture. It's not part of the
sampler state.

We set the swizzle when it is a feature, but always to RGBA.

Also, highly speculative that this may improve ANGLE D3D11 ES3
performance compared to ES2.

Bug: skia:10644
Change-Id: I8877afc3043c5ddaafd26ea9f9bd372303328c71
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/313682
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
14 files changed