asfdemux: fix seeking in push mode
asfdemux: fix build on C89 compilers
ext: Use new flush vfunc of video codec base classes and remove reset implementations
mpeg2dec: Fix mpeg2_reset() calls
x264enc: minor clean-up

We requires x264 build >= 120 now.
configure: fix x264 version checking

x264 releases carry pkg-config versions such as 0.120.x
(literallly with an 'x'). Checking for x264 >= 0.120.0 will
then fail, so we need to either check for > 0.119.0 or
for >= 0.120 to make this work right.