Treat color target format with "float" capabilities as
blendable format and validate when blending is enabled.
Add helpers for checking float16 texture values.
Bug: dawn:726
Change-Id: Icf8c0182e5e9a13523970c84b5af91f395a089af
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/57744
Commit-Queue: Shrek Shao <shrekshao@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>