Displaying background textures on a 120 Hz display is slower than expected

[hubbardp]

Pavi reports that when she uses org.janelia.background to display a set of background textures at 120 Hz (i.e., "durationSecs": 0.0083) on a 120 Hz external display then the total elapsed time is longer than expected, by several seconds.

[hubbardp]

Addressed by the changes on the branch "background-changer-faster":
https://github.com/JaneliaSciComp/janelia-unity-toolkit/tree/background-changer-faster