qtmux: Un-merge the last two stsc entries after serializing
qtmux: Un-merge the last two stsc entries after serializing

The last entry will most likely get new samples added to it in "robust"
muxing mode, changing the samples_per_chunk and thus making it wrong to
keep the last two entries merged. It will run into an assertion later
when adding a new sample to the chunk.

Thanks to gdiener@cardinalpeak.com for the analysis of the bug and
proposal for a solution.

stfl/gst-plugins-good,stfl/gst-plugins-good,stfl/gst-plugins-good,stfl/gst-plugins-good