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/Tex/TextureUnit.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <AK/IntrusiveList.h>
 #include <AK/OwnPtr.h>
 #include <LibGL/Tex/Texture2D.h>
 
@@ -35,6 +36,9 @@ public:
     bool texture_cube_map_enabled() const { return m_texture_cube_map_enabled; };
     void set_texture_cube_map_enabled(bool texture_cube_map_enabled) { m_texture_cube_map_enabled = texture_cube_map_enabled; }
 
+    IntrusiveListNode<TextureUnit> m_bound_node;
+    using BoundList = IntrusiveList<&TextureUnit::m_bound_node>;
+
 private:
     mutable RefPtr<Texture2D> m_texture_target_2d { nullptr };
     mutable RefPtr<Texture> m_currently_bound_texture { nullptr };