override the background color of an EnvironmentCamera

[stephengold]

Currently EnvironmentCamera copies its backGroundColor field from the application's default ViewPort. However, an application may have multiple view ports. In such applications, the default may give unintended results.

This issue proposes adding a setter to alter the background color of an EnvironmentCamera.

[stephengold]

Fixed in "master" branch at 4fdee8f .