ImageryLayer Gamma calculations are wrong

Compare behavior between the Adjustment demo in [1.52](https://cesiumjs.org/releases/1.52/Apps/Sandcastle/index.html?src=Imagery%20Adjustment.html) and [1.53](https://cesiumjs.org/releases/1.53/Apps/Sandcastle/index.html?src=Imagery%20Adjustment.html). You will have to switch to a different baselayer, per #7706. The Gamma slider in 1.52 makes the image slightly darker when moved to the left (say, 0.9) and slightly lighter when moved to the right (say, 1.1). In 1.53, any value over about 0.5 makes the image lighter, except 1.0 exactly, which behaves as you'd expect it to. Experimentally, 0.45 looks almost identical to 1.0.

Something introduced in 1.53 must have changed the calculation for gamma, and it no longer behaves as intended.

Reproduce the Gamma slider behavior in the 1.52 and 1.53 Adjustment demos, switching baselayers as noted and comparing values around 0.45, 0.9, 1.0, and 1.1. Trace the ImageryLayer gamma calculation and verify that the corrected behavior matches the 1.52 comparison, with values below or above 1.0 adjusting brightness in the expected direction.

Written by the indexing model from the issue text.