Fixed `playdemo' does not work on files outside of the working directory.

[jameds]

Running `playdemo' on a file that's outside of the working directory fails with:

ERROR: Failed to read file 'blah.lmp'.

This only affects systems that use srb2home, i.e. not Windows.

 

[Monster Iestyn]

Just made a merge request to this fix myself: https://git.magicalgirl.moe/STJr/SRB2/merge_requests/221

(yes I know you made one too, but I think this is probably the proper fix for this particular problem?)