audio/format: reformat
audio: replace af_fmt2str_short -> af_fmt_to_str

Also, remove all af_fmt2str usages.
audio/format: convert format macros to enum, drop NE suffix

Turn the sample format definitions into an enum. (The format bits are
still macros.) The native endian versions of the new definitions don't
have a NE suffix anymore, although there are still compatibility defines
since too much code uses the NE variants.

Rename the format bits for special formats to help to distinguish them
from the actual definitions, e.g. AF_FORMAT_AC3 to AF_FORMAT_S_AC3.
af: remove a pointless macro

The code should be equivalent; a compatibility macro definition is left.
(It should be mass-replaced later.)
vdpau: drop dummy render function

This was supposed to handle preemption better. I still think the current
state isn't very nice, since the decoder can "accidentally" call the
previous render function after preemption (instead of calling the
reloaded function), so there might be issues. But all in all, this
dummy_render function is a bit confusing, and still not entirely
correct, so it's not worth it.
audio: fix bogus audio format comparison