Support for 9 and 10-bit encoding

Output bit depth is specified on compilation time via --bit-depth.
There is currently almost no assembly code available for high-bit-depth modes, so encoding will be very slow.
Input is still 8-bit only; this will change in the future.

Note that very few H.264 decoders support >8 bit depth currently.
Also note that the quantizer scale differs for higher bit depth.  For example, for 10-bit, the quantizer (and crf) ranges from 0 to 63 instead of 0 to 51.