Native bindings to the ARB_explicit_uniform_location extension.

This extension provides a method to pre-assign uniform locations to uniform variables in the default uniform block, including subroutine uniforms. This allows an application to modify the uniform values without requiring a GL query like GetUniformLocation, GetSubroutineUniformLocation and GetSubroutineIndex.

Requires OpenGL 3.3 or ARB_explicit_attrib_location. Promoted to core in OpenGL 4.3.