Uniforms hack

[adamstark]

I raised an issue around setting uniforms using OpenGLGraphicsContextCustomShader - full context on the JUCE forum:

https://forum.juce.com/t/requested-improvements-to-opengl-shader-support-in-juce/27422

This solution allows uniforms to be set using a std::function. It involves a copy of a function which should be improved but this should be the basis of a workable solution.