Texturing in app

[edemaine]

Let's add the ability to upload a custom PNG, or use an in-built "real paper" texture, to texture onto the folded surface, perhaps registered to the topmost leftmost vertex of the crease pattern by default. In SVG input format, we could even support an <image> tag (with inline data).

As suggested in #50, we could also support exporting the folded surface as OBJ with texture.

[jakobhenner]

What an amazing tool!

I wish I could actually contribute, but any news on texture support?

I tried to use THREE's TextureLoader and it does have an effect - the main color from the texture shows but it seems like the texture is not mapping correctly to the geometry.