movenc: Grow the frag_info array in chunks

Previously it was grown one element at a time, which leads to
excessive reallocations.

Bug-Id: 525
avio: Add a function for reusing a dyn buffer

This avoids having to free and realloc a growing array for cases
where a similar buffer is used repeatedly.

Bug-Id: 525
movenc: Use avio_reset_dyn_buf for fragment data payloads

This avoids reallocations for subsequent fragments (when all
fragments are roughly the same size).

Bug-Id: 525