Change texture creation to use nearest filtering. Fixes several reftests

due to the different texture sizes that are created now with the removal
of the quadtree code.
Technically speaking, don't we want to check that `transform` is a 2D affine transform and there is no skew or nonrectilinear rotation? I'd feel cruel asking you to implement this, given that it doesn't matter right now, but can you at least add a FIXME to that effect?