We had assumed in Context in various places that negative pixel values
would be clamped to zero, which is incorrect for floating-point color
attachments.

TODO: on the assumption that there is value in these blend optimizations
themselves, we should split the context blend state to be
per-attachment. This would give us the `independentBlend` optional
feature, and also allow us to keep the optimization for those
attachments which do have clamping in a mixed MRT scenario.

Bug: b/132433217
Test: dEQP-VK.pipeline.blend.format.r16*
Change-Id: I5434475aba5d21a8c8342b42cccc575b517f0900
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31050Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>