The previous allocation increment of 16384 meant that the cluster
array was allocated for 0.6 MB initially, which is a bit excessive
for cases with fragmentation where only a fraction of that ever
actually is used.

Therefore, start off at a much smaller value, and increase by
doubling (to avoid reallocating too often when writing long
non-fragmented mp4 files).

Bug-Id: 525
Signed-off-by: Martin Storsjö <martin@martin.st>