shaders

[PSteinhaus]

ggez currently features three functions allowing you to use custom shaders when drawing:

• set_shader
• clear_shader
• and use_shader

It should be possible to implement them using miniquad, but since I'm relatively inexperienced when it comes to shaders (and using shaders with miniquad) I leave this open for now for anyone willing to help.

Once the functionality is implemented porting the two examples shader.rs and shadows.rs should be pretty straightforward (at least for shader.rs, not sure whether the shaders in shadows.rs can be written to work in glsl100).

[PSteinhaus]

Ok, I managed to struggle through it and I'm pretty happy with the result!: 977b0d5

Now all that's left is trying to port shadows.rs, though I'm still not sure whether this is possible in glsl100; we'll see.