DxVA2: remove vlc_object_t back pointer
vaapi: use driver name (rather than VA-API ABI version) as description
va: make the description pointer const

4 out of 5 plugins expect it that way and the owner should not modify
the string anyway.
contrib/gcrypt: fix compilation on x86_64/Darwin with clang

Patch is pending for upstream submission
Add basic A_QUICKTIME audio support in MKV

Fix partially #11038 since seek is still broken