Small fixes

So, I had a few hours to spare and fixed some little details. I didn't add any gameplay stuff, though.

This should fix: 

• Resolution problems (It's 4:3 by choice)
• Collisions (hopefully)
• Heart pick-ups not jumping
• A crosshair instead of the default mouse pointer
• Other stuff I forgot

I couldn't test the Linux and MacOs builds, so I'm not sure if they even work.

EDIT: Linux build doesn't seem to work on my computer. But it's a Unity problem, since any other game I build on 2019.X seems to have the same issue (pink screen).