[2023-08-22Update] add BGM, fix bugs about fullscreen mode

Hello everyone :)

Finally I put BGM into the game. I found that there is no better choice than using the BGM of Alan Becker's origin video (by Scott Buckley), so I picked a few clips from it as BGM.

Also, thanks to the feedback from @rainbow-bytes (Indie game inspired by Alan Becker's "Animation vs Math"! - Release Announcements - itch.io), I fixed the bug of the fullscreen button. Now you can enter the fullscreen mode normally.

Bx007,

2023-08-22