Improve -nomousegrab mouse input handling

[Lgt2x]

Pull Request Type

• GitHub Workflow changes
• Documentation or Wiki changes
• Build and Dependency changes
• Runtime changes
  • Render changes
  • Audio changes
  • Input changes
  • Network changes
  • Other changes

Description

Minor fixes to #695 (@tophyr ):

• event->motion.[x/y]rel is float, so store DDIO_mouse_state.dx as float to avoid losing precision. This is especially needed when mouse is not grabbed by the window (-nomousegrab option) where xrel/yrel does not only have integer values.
• Remove broken legacy code supporting -nomousegrab mode. Position is still not the same between the actual OS cursor and the in-game cursor, but it is better than it was before.