Fix cursor state toggling (during medleys)

[dgruss]

As reported in #1344, there were some cursor state toggling bugs in the game in the context of medleys.

With this PR we

• set singing-screen cursor hiding based on the current screen state instead of toggling the previous state
• add an explicit cursor screen-hide mode for cases where the current screen is still ScreenSing but the cursor should follow normal visibility rules
• keep the singing help popup cursor override working without manually mutating Cursor_HiddenByScreen

Fixes #1344.

Bugs beyond #1344:

• starting the game in full screen mode and switching to windowed mode --> mouse cursor remains invisible
• starting the game in windowed mode and switching to full-screen mode --> mouse cursor remains visible

I fixed these two cases as well as a second commit