This PR adds several tests to GTest for testing blend states
and immediate mode. The immediate mode test are basic and will
be extended in the near future.
The tests have been developed in order to get the first pixel on
screen for the Vulkan backend. In the first phase of this goal we
had to validate that everything was working as expected without
being able to validate it by starting Blender.
Pull Request: https://projects.blender.org/blender/blender/pulls/107834
