Add support for RG float textures

Adds kRG_float_GrPixelConfig.

Also removes default labels from switches on GrPixelConfig, in order
to help guide future enumerals to handle them properly.

BUG=skia:

Change-Id: Ie80b9413b4002b666df3ef1a7a8ea4c9c29ce43b
Reviewed-on: https://skia-review.googlesource.com/8226
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
8 files changed