Support uniform arrays

[not-fl3]

Before this PR, it was impossible to create macroquad material with an array as an uniform type.

Now it is possible:

    let mat_with_array = load_material(
        ShaderSource::Glsl {
            vertex: VERTEX,
            fragment: FRAGMENT_WITH_ARRAY,
        },
        MaterialParams2 {
            uniforms: vec![UniformDesc::array(
                UniformDesc::new("test_color", UniformType::Float4),
                10,
            )],
            pipeline_params,
            ..Default::default()
        },
    )

To reduce an impact of a braking change, load_material signature changed:

pub fn load_material(
    shader: crate::ShaderSource,
-     params: MaterialParams,
+     impl Into<params: MaterialParams>,
 ) -> Result<Material, Error>;

Most of the old code using load_material should keep working, while now it is possible to use MaterialParams2, the same struct, but with a complete UniformDesc. Complete UniformDesc now allows loading materials with arrays as a uniform.

A custom_material example got updated to show how to use uniform arrays.