Since av_gettime() is used in a number of places where actual
real time clock is required, the monotonic clock introduced in
ebef9f5a would have consequences that are hard to handle. Instead
split it into a separate function that can be used in the cases
where only relative time is desired.

On platform where no monotonic clock is available, the difference
between the two av_gettime functions is not clear, and one could
mistakenly use the relative clock where an absolute one is
required. Therefore add an offset, to make it evident that the
time returned from av_gettime_relative never is actual current
real time, even though it is based on av_gettime.

Based on a patch by Olivier Langlois.
Signed-off-by: Martin Storsjö <martin@martin.st>