This already revealed some places where implicit conversions were done, including some where its use was nonsense (MouseInput.cpp).
The changes to the Intent class were necessary to keep things working, and this splits things up more neatly.
Define see-through paint colours in Paint.h for global use
Color staff sprite button
Change ride->is_visible bool to ignore_invisible_flag bool
Add shortcut to open transparency options (CTRL+T)
- Changes all references of `VIEWPORT_FLAG_INVISIBLE_PEEPS` to `VIEWPORT_FLAG_INVISIBLE_GUESTS` and `VIEWPORT_FLAG_INVISIBLE_STAFF`
- Updates Invisible People menu item and shortcut.
- Added new viewport flags into the `viewport_set_visibility` function
- Invisible options only work if see-through is activated as well
- Added see-through supports
- Regression: Exclude ride from invisible no longer works (ride is still see-through)
