[QML Map] MapPoygon alpha color used also for border color in MapboxGL

Use the attached QML file to reproduce 2 bugs related to MapPolygon colors on MapboxGL:

1. MapPolygon alpha color is used to determine if the polygon should be drown on the map or not. Because of this it is not possible to draw only the border of the polygon without filling it. Also the color alpha is used for rendering the border.

2. Changing the color of a MapPolygon has no effect after the polygon was added to the map.