ismindex: handle time discontinuities and nonzero start time

The input file may not have consistent start times, stream durations and
chunk durations. This patch at least removes negative durations that
make chromecast unhappy, and correctly sets starting time on chunks so
that the split (or .ismf) outputs match the manifest.

Signed-off-by: Martin Storsjö <martin@martin.st>
Merge commit '00431bf874e1044b01e09a2266ef85d4ff8d44cc'

* commit '00431bf874e1044b01e09a2266ef85d4ff8d44cc':
  ismindex: handle time discontinuities and nonzero start time

Merged-by: Michael Niedermayer <michaelni@gmx.at>
build: simplify libwebp check

Tested (compilation and runtime) with libwebp 0.2.0, 0.2.1 and 0.4.1.
avformat/network: move sockaddr_union after sockaddr_storage compatibility code

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>