Don't globally set float precision to mediump, only fragment
shaders need that and defining it for vertex shaders causes
tilemap cracks.
Also manually define low precision for variables that hold
color / alpha values.
This implementation is also heaps better than the old
one as it doesn't use a (differently sized) aux texture,
meaning the Bitmap discards its old texture and aquires
one of same size, making reuse through the TexPool a
lot more likely. It also saves on the aux texture blits
and binding switches.
As the setup / resource acquisition far outweighs the
actual rendering cost, operation time is relatively
constant no matter how many divisions are used.
Jonas Kulla