fix base64 errors

[xBZZZZ]

geometry dash sometimes adds up to 3 bytes of garbage to end of CC*.dat files

so I changed it to round down multiple of 4 (length of base64 encoded string is always multiple of 4)

that solution worked for me in https://gdccdated.glitch.me/index.xhtml (it loads save file every time I tried without random errors)

also I think this fixes #5