qtdemux: When receiving a DISCONT buffer that does not point to a sample, remember...

qtdemux: When receiving a DISCONT buffer that does not point to a sample, remember the offset

And don't just reset everything. This makes sure that we can continue to
handle data in the following scenario:

moov: discont
moof: discont
mdat: continuous

Previously this would fail because the offset would be the accumulated offset
from moov and moof at the mdat position, while the buffer offset might be
something completely different.