Updated the list of supported input formats, added note about 16-bit
signed format (no fractional bits).

Make a note about 16-bit signed format - the function assumes that
values have no fractional bits (so 16-bit disparity from StereoBM
and StereoSGBM cannot be directly used!)

This is to achieve parity with the CPU equivalent.

FreeBSD compilation fix

MSMF expects a bottom-up video frame, so the buffer pointer must point to the end of the data and the stride needs to be negative