This passes all the non-sparse BC6h format tests in dEQP.

Bug: b/151203718
Tests: dEQP-VK.*bc6*
Change-Id: I5dacbc07bb54ff4fc384db974feaf7cfd0055e96
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/45571
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Sean Risser <srisser@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>