Fix bug in find_closest_blockdim_2d()
Fix bug in find_closest_blockdim_2d()

When user specifies rate as a desired bpp, then codec tries to find the legal block size with a closest bpp. Original code has treated some legal block sizes as illegal.

ARM-software/astc-encoder,ARM-software/astc-encoder,ARM-software/astc-encoder,ARM-software/astc-encoder