There can be cases where there are multiple SEI payloads within a
single SEI NAL. In the particulkar case where the payload comprises
exclusiely of FGC data, the size of the NAL can exceed the size
of the 'dynamic bitstream buffer' which is used to pass the NALU
onto its appropriate parser.
This commit adds 'imvcd_bitstream_buf_realloc' which re-allocates
the 'dynamic bitstream buffer' such that any arbitrarily sized
NALU can be stored without a heap overflow.
Bug = ossfuzz:64286
Test: svc_enc_fuzzer
37beb97292