Add support for shader subroutines

[j-o]

See https://www.khronos.org/opengl/wiki/Shader_Subroutine

glUniformSubroutinesuiv must be called after every call to glUseProgram to (re-)activate subroutines. Since subroutines have not much in common with normal uniforms, I suggest a special integration: store a list of selected subroutine indices by shader type inside Program, expose corresponding get/set functions and call glUniformSubroutinesuiv in Program::use() if the list is non-empty. (Plus the same for ProgramPipeline)

[scheibel]

Sounds reasonable. Do you have a preliminary implementation?

[j-o]

Nope, haven't started this.