Allow texture type

[mdsumner]

to avoid some need for rgb, also see pyramids in hypertidy/anglr#150

[mdsumner]

it definitely works, have to be very careful to have

• no textype property and RGB png (it's assumed)
• no mismatch between textype and layers in the png

otherwise it crashes R

[mdsumner]

anglr's a bit of a mess for this, I think we'll move this to textures and fix from there