Missed variables in window_to_world method (#46)

* fix: Missed variables

* ref: Unused variable

Refresh camera list on update() and before use of get_cameras()

Use first camera if none is provided

Better handling of viewport defaults

Fixes #44

Always setup blend mode and all relevant states even when there is no…

… camera

Moved gui render outside camera loop

Added support for camera viewport (#41)

* Added viewport settings for camera

* Updated screen_to_world to take into account viewport

* Added multi camera support

Handle gui adjust modes correctly

Don't change the z value when calling screen_to_world

Fixes #34

Maybe fix trouble with bounds (#37)