Custom shaders no longer possible

[Jouwee]

Custom shader support was added in #265. However, since then, the TexturedColored shader has been created bbut not externalized, so now it's no longer possible to construct GlGraphics with custom shaders.

   --> src/main.rs:202:76
    |
202 |         gl: GlGraphics::from_pieces(Colored::new(glsl), test_shader(glsl), TexturedColor::new(glsl)),
    |                                                                            ^^^^^^^^^^^^^ use of undeclared type TexturedColor```