examples/viewer:Constant GPU usage even when no animation is running

[ai-and-i]

When opening rive files in examples/viewer, I noticed that it constantly shows pretty high GPU usage, even when the rive animation is not running. In comparison, opening the same files in rive-flutter shows near-zero GPU usage in the same situation. When the animation is running, the GPU usage of rive-flutter and rive-rs viewer are comparable.

This is quite problematic for battery-powered embedded devices, which is a very attractive use case for rive-rs.