Seems to work; note that idWaveFile is only ever used in idSoundSample::Load()
As stb_vorbis doesn't support custom callbacks for reading, I feed it
the full .ogg files as a buffer. Shouldn't make much of a difference
though - either the whole file is decoded on load anyway (so the buffer
is freed after decoding, or it's streamed, but in that case the old code
also kept the whole ogg file in memory by using idFily_Memory.
I also added warning messages in places where calls to stb_vorbis_*()
can fail, where there were none in the equivalent libvorbis code.
This reverts commit 01ac144b09.
Looks like this broke the build on some systems, because some
package managers pack SDL2Config.cmake and others pack sdl2-config.cmake
in their SDL2 development packages (for some reason, SDL2 seems
to ship both in their source, and they appear to be incompatible).
Shipping our own FindSDL2.cmake might be unclean/ugly/whatever, but
at least it works (most of the time? at least it appears to work better
than not shipping it)
