av_memcpy_backptr: optimise some special cases

- Add special cases for offsets of 2, 3, or 4 bytes.  This means the
  offset is always >4 in the generic case, allowing 32-bit copies to
  be used there.
- Don't use memcpy() for sizes less than 16 bytes.

Signed-off-by: Mans Rullgard <mans@mansr.com>