st/nine: Add D3DFMT_DF16 support

st/nine: Add D3DFMT_DF16 support

This depth buffer format, like D3DFMT_INTZ, can be used to read
the depth buffer values when bound to a shader.

Some apps may use this format to get better performance when
they don't need the precision of INTZ (24 bits for depth, 8 for
stencil, whereas DF16 is just 16 bits for depth)

We don't add support for DF24 yet, because it implies support
for FETCH4, which we don't support for now.

Reviewed-by: Tiziano Bacocco <tizbac2@gmail.com>
Signed-off-by: Axel Davy <axel.davy@ens.fr>