Release v0.3.5

- VpxPacket now exposes pts() and duration(), echoed back from each encode() call,
  so callers can correlate encoded packets with input frames and compute mux timing
- Both values are read from the native vpx_codec_cx_pkt_t.data.frame struct; no
  overhead beyond what libvpx already provides
- New VpxEncoderConfig fields: BitDepth enum (8/10/12-bit), inputBitDepth,
  resizeAllowed, and minKeyframeDistance — all wired to the corresponding libvpx
  FFI setters
- New round-trip tests for PTS/duration in Vp8EncoderTest and Vp9EncoderTest