The Vulkan spec states that "blendEnable [...] must be VK_FALSE if the
attached image’s format features does not contain
VK_FORMAT_FEATURE_COLOR_ATTACHMENT_BLEND_BIT"

Previously we would silently ignore blending of integer formats. It was
determined to be undefined behavior in https://gitlab.khronos.org/vulkan/vulkan/issues/2098

Bug: b/155147929
Change-Id: I01500d8c39d7f2c9a484944b4a93c6004e938c05
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/45708
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>