GLRenderer seems to try to ensure only 0..N TIUs are used in a draw call using N textures. I think this is unnecessary as GLES 2.0 allows any combinations of available TIUs to be used in a draw call.

For example RenderPassDrawQuad contains up to 4 textures of different semantics:
0. filter_image/contents_texture
1. mask 
2. backdrop
3. original_backdrop
It would be simpler to always bind backdrop texture to TIU 2 regardless of whether mask sampler is required. 

Just for reference newer GLSL allows layout(binding=) qualifier on samplers for such purpose and I have the impression that most game engines pre-assign TIUs by semantics in the build process.