Converter outputs misleading filenames

[nekno]

When using convert --discard the RPU is converted to the desired format and the converter outputs a single-layer BL+RPU track, discarding the EL.

When a custom output filename isn't specified with -o, the converter always outputs a file named BL_EL.hevc by default.

Since the intended output with the --discard option is a single-layer BL+RPU track, it's confusing to see EL in the name, leaving the user to wonder if the --discard option worked.

It would be helpful and more clear if the default output conveyed its contents:

• BL_RPU.hevc with --discard and BL_EL_RPU.hevc without, or
• BL.hevc with --discard and BL_EL.hevc without, if you're opposed to indicating the RPU is included, as seems to be the case in other filenames