Avoid reopening file on every FileIO read/write

[flyngmt]

FileIO::fileRead/fileWrite open and close the file on every call, which causes noticeable slowdowns on Card::format().

Takes like a minute on an HDD and 10-15 seconds on an SSD to create the .raw file on the initial run, fixes that by just keeping the file open instead.

[encounter]

Thanks for the PR! It looks like this was explicitly introduced in #77 to avoid locking the file while the game was running. However, I agree that reopening the file on every write is not ideal. Maybe we can keep the file open only while actively writing to it, and close it afterwards.

[flyngmt]

I didn't realize it was such a new change, lol.

Yeah, that's probably the best way then. Current one helps the development but most end users will think game just crashed especially if they are using an HDD.