Got sucked into refactoring a bunch of stuff again. ;_;
But it's better to pass the output directory and resource basename
and let each converter combine or mangle those as needed. The most
common special case is producing multiple numbered graphics from
a single source file.

While at it, also cleaned up bitmap and metadata object instantiation
during graphic conversions, so it's only done after parsing the file
header (if possible). If there's a header mismatch (wrong converter),
there was no point reserving memory that just gets immediately freed.
This won't make a noticeable speed difference, but it's cleaner.