clean up the temporary component buffers

combined JPEG 'receive' and JPEG 'extend', since baseline always extends everything it receives.

returns 0..31 for the highest set bit

mallocs with size overflow checking

extract an arbitrarily-aligned N-bit value (N=bits) from v, and then make it 8-bits long and fractionally extend it to full full range.

returns STBI_rgb or whatever, 0 on error

This one might be too, can't remember.

return 1 if the sum is valid, 0 on overflow. negative terms are considered invalid.

@OPTIMIZE: provide an option that always forces left-predict or paeth predict