This should replace .dispose member in video_frame in future.

This allows to run UltraGrid in separate processes for sending and
receiving at the same port.

Do not occupy audio ports when audio not used and the same applies for
video.

Wrong computing of offset when video height is not divisible by
cpu_count*2.

Fixes problems with GCC 4.6

Use all cpu cores for colorspace transfomrations (was 1/4). The reason
for previous was when compressing multiple tiles (in parallel) we would
use 4-times more threads than cores. That is, however, usually not the
case.