Required with later discs (? 2016 -):
Some patches corrupt good parts of stream.

- Support stream processing without full conversion table.
- Querying masks from the VM is not yet implemented.
  => Testing requires injecting masks with BDPLUS_CONVTAB environment variable.
  - Getting correct masks from VM is easy to implement, but requires mode 1 VM config.
    - Mode 2 seems to use different masks.
    - Mode 2 masks may also change between playback devices / sessions (?).
    - Masks should be queried from VM in sync with playback (BD-J ?).

Caching the masks would make starting next playback sessions faster, and
allow playback without menus.
Caching masks requires < 10 KB space (full tables use > 1 MB).

If no cached convtab filename was passed via the BDPLUS_CONVTAB env var, find
and eventually use a cached convtab located (in this order) in the home or
system bdplus config and cache dirs, if the cache file contains the discs
media key in it's file name.

Fixes:

src/libbdplus/internal.c: In function ‘bdplus_run_convtab’:
src/libbdplus/internal.c:337:53: warning: unused parameter ‘num_titles’ [-Wunused-parameter]
 int32_t bdplus_run_convtab(bdplus_t *plus, uint32_t num_titles)
                                                     ^~~~~~~~~~

VM-generated table can be overridden with cached table by using
environment variable BDPLUS_CONVTAB.