Partially implement texture generation in scene

It's incomplete so far because I need to refer to some old code
for a sec.
Draw the texture with triangles

It doesn't quite work yet. The triangles are being drawn but they're
black.
Correctly obtain width and height values

Now the textures draw correctly onto the triangles, however it also
becomes very slow to render. This can probably be fixed by subsampling
in the same way that it was done with SceneTriangulation.
Subsample the loaded textures

Unfortunately it doesn't seem to speed up the rendering very much.