Add support for VkLogicOp

MoltenVK currently does not support the VkLogicOp member of the VkPipelineColorBlendStateCreateInfo structure, for the understandable reason that Metal has no equivalent functionality. However I make extensive use of AND, OR and XOR logical operations in my applications and the loss of this feature would be a very serious problem for me (currently I am using glLogicOp in OpenGL and hoping Apple doesn't drop OpenGL support in MacOS any time soon!).

It has been suggested that a workaround might be possible in MoltenVK by implementing the logical operations in a shader, is this something on the radar?