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

exp(x) = lim(1+x/n)^n // n=10 */

exp(x) = lim(1+x/n)^n // n=10 */

TODO: could specialize these to not use AsInt64() if that saves extension ops in generated code, and use a faster op than ReadInt64.

Helper to see if the identifier in a buffer has the expected value.

Offsets initially are relative to the end of the buffer (downwards). This function converts them to be relative to the current location in the buffer