Add D2V support (back)

[RivenSkaye]

This PR fixes the lost d2v support, as not everyone uses DGIndexNV (e.g. those with AMD GPUs).

• Adds IndexExists.PATH_IS_D2V and IndexExists.D2V_EXISTS, mirroring the behavior for the dgi types
• Adds checking for d2v
• Replaces the nonexistent core.dgdecode.DGSource with core.dgdecode.MPEG2SOURCE
  • The first argument of this must be the d2v, but the remaining args and kwargs due to the differences to dgdecodenv
• Replaces check_output with check_call since the output of the command is irrelevant. Unless nvidia-smi sets a non-zero exit code when called without arguments. This needs testing on another system

[RivenSkaye]

Marked it as a draft due to the differences between dgdecode and dgdecodenv, will fix it later on

[LightArrowsEXE]

Ideas we talked about:

• Reverse the percentage check (d2v's mark the % VIDEO, not FILM. To resolve this, we can simply do abs(PERCENTAGE - 100), I think? Need to check whether it's consistent at the same place, and whether any values change given different types of files.
• Different d2v indexer than dgindex? Currently it has trouble indexing certain file types (like mkv), which isn't ideal when you explicitly need it (like for remuxes, for example). Not sure I wanna add explicit support, or just tell users to call the functions directly themselves
• d2vsource as source filter for d2v files instead of dgdecode
• dgdecode with d2v.ApplyRff