libavcodec/libopenjpegenc: enable lossless option, remove layer option, and improve defaults

1. limit to single layer, as there is no current support for setting distortion/quality of multiple layers
2. encoder mode should be kept at default setting (0)
3. remove fixed_alloc parameter from context : seldom if ever used, and no way of properly configuring at the moment
4. add irreversible setting, to allow for lossless encoding. Set to OpenJPEG default (enabled)
5. set numresolution max to 33, which is the maximum number of allowed resolutions according the J2K spec

Signed-off-by: Michael Bradshaw <mjbshaw@google.com>
