LibGL: Only pass bound texture units to rasterizer

Before, `SoftwareRasterizer` was iterating over all 32 possible texture
units for each fragment and checking each if they're bound to a texture.

After this change, an intrusive list containing only texture units with
bound textures is passed to the rasterizer. In GLQuake, this results in
a performance improvement of ~30% (from 12 to 16 FPS in the first demo)
on my machine.

Userland/Libraries/LibGL/SoftwareGLContext.h

@@ -234,6 +234,7 @@ private:
     HashMap<GLuint, RefPtr<Texture>> m_allocated_textures;
     Array<TextureUnit, 32> m_texture_units;
     TextureUnit* m_active_texture_unit { &m_texture_units[0] };
+    TextureUnit::BoundList m_bound_texture_units;
 
     SoftwareRasterizer m_rasterizer;