Custom fragment shader support

[QuantumBadger]

It would be possible to add an easy-to-use API for registering custom fragment shaders, and drawing triangles using those shaders.

The corresponding vertex shader could be dynamically generated to support passing an arbitrary number of attributes, and the ability to pass uniforms could also be exposed.

[blaumeise20]

Please add this, I really need it for rendering a large grid on my game!