Add fragment shader interlock to core OpenGL 4.6

Put ARB_fragment_shader_interlock in core OpenGL 4.6.
https://www.opengl.org/registry/specs/ARB/fragment_shader_interlock.txt

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.)
https://www.opengl.org/registry/specs/KHR/blend_equation_advanced.txt