Add note about uninitialized internal buffers
- Added `peek_uxx`, `skip_fast` and `write_fast_uxx` methods.
- Renamed some `read_uxx` methods as `read_uxx_as_uyy`.
- Added I/O positions.
+- Added extra fields (uninitialized internal buffers) to structs.
- Tweaked how marks and limits work.
- Supported animated (not just single frame) and interlaced GIFs.
- Marked some internal status codes as private.
