test whether an EnvironmentCamera is busy

[stephengold]

EnvironmentCamera has the capability to queue up multiple snapshot jobs. However, it is of limited utility because the camera's position, background color, and image format aren't job-specific. Therefore, they shouldn't be modified while a job is running. To generate multiple snapshots from a single EnvironmentCamera, it's important to know whether there are active jobs. This is difficult because the jobs field is private.

This issue proposes adding an isBusy() method to EnvironmentCamera so that applications can avoid modifying the camera's position, background color, and image format if the EC is busy.

[stephengold]

Implemented this, but it's not as useful as expected. Most of the latency for generating a LightProbe is in calculating spherical harmonics and such. The EnvironmentCamera is busy for just one frame.