True integer formats should not use blend modes.
This cl explicitly disables any blend operation
on render targets using integer formats.

Bug: b/153203552
Change-Id: Ia756953c61caef8e32e55aa31126044973ae1870
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43548
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>