Implement texture filtering

[bvssvni]

Depends on PistonDevelopers/texture#32.

Change Texture into:

pub struct Texture {
    pub texture: Texture2d,
    pub min_filter: MinifySamplerFilter,
    pub mag_filter: MagnifySamplerFilter,
}

• Map TextureSettings::get_mag for MagnifySamplerFilter
• Add a function that maps TextureSettings::get_generate_mipmaps, TextureSettings::get_min and TextureSettings::get_mipmap for MinifySamplerFilter
• If TextureSettings::get_generate_mipmaps is true, call Texture2d::with_mipmaps
• In the impl of Graphics::tri_list_uv, use .sampled() to set .mag_filter and .min_filter.