Optimizations for make_texture
1. New options for the make_texture configuration: "maketx:read_local_MB"
   can change the threshold for using ImageCache; "maketx:forcefloat"
   can disable the convert-to-float step; "maketx:hash" can disable
   computing the SHA-1 hash.
2. More efficient contiguize() by breaking out the case where it's
   contiguous from pixel to pixel, but not from scanline to scanline.
3. Special case the resize step when it's reducing by exactly 50% using
   box filter (just averaging groups of 4 pixel) and the types are equal
   and a few other common preconditions -- faster than the general resize.