Add fragment shader interlock to core OpenGL 4.6

Put ARB_fragment_shader_interlock in core OpenGL 4.6.

Extend ARB_fragment_shader_interlock functionality to allow using predefined blend modes from KHR_blend_equation_advanced.
Allowing more compact, easier to use and performant shaders.
(This would also make the resulting shader interlock + blend modes extension a superset of blend equation advanced.)