Compiles but untested.

NOT INTENDED FOR MERGING

This involved removing any logic performed in loadsave for loading saved games and moving them to callback routines for all uses.

Reduces padding:
* In `rct_viewport` from 5 to 1
* In `rct_ride_entrance_definition` from 4 to 0

Redeclaring the same typedef is not legal in C < 11, but a
forward-declaration works just fine for having a pointer to struct. No
changes needed in .cpp, as it includes the `typedef` from `object.h`
(via `scenario.h`).

Only reset the ride viewport if it the window has actually been resized and maintain the viewport flags.

By using a union instead of a struct var_494 and var_496 where ontop of the same location this meant var_496 when written to would overwrite ride_colour that was also in a union with var_494/496. This in turn would cause the wrong ride_colour to be overwritten when using the colour picker.

This still doesn't work as well as I'd like yet, most notably when zooming out.

This provides an option to set a limit on the number of windows that can
be open. Originally, the game had somewhere around 12, but was bumped up
to 64 for OpenRCT2. While this provides some value, this is for people who
prefer less windows at a time, and also for people with older computers.
The limit can be set anywhere between 8 and 64, (8 being chosen to still
preserve in-game toolbars that are counted as windows, and 64 to not
overshoot the maximum hard-coded value). This can be set in both options
and the in-game-console.

This makes sure every function is properly declared, which includes
proper `extern` wrappers.