Store texture coordinates for each slice
instead of assuming 0,0,1,1, and generate
overlapping slices to allow for proper mipmaps.
This almost fixes trilinear filtering with
sliced textures.
Instead of uploading a texture once per filter, ensure textures are
uploaded as little as possible and use samplers instead to switch
different filters.
Sometimes we have to reupload a texture unfortunately, when it is an
external one and we want to create mipmaps.
