Atmosphere gets too bright with HDR turned on

[jjspace]

This is based off an observation from @ggetz while we were working on #12126

Atmosphere seems really bright when HDR is on. Check to make sure we're not accidentally doubly applying tonemapping for the atmosphere when HDR is on. Maybe verify if this issue was present with ACES tonemapping or not.

HDR off

HDR on

Notice that the atmosphere does get "brighter" when HDR is on. I notice it most as the blue part shrinks to almost nothing.

Another weird part of this is that if you crank the brightness way up you can see the total height of the atmosphere including the fully transparent pixels. That might be a separate bug but it also reveals that with HDR off those pixels are semi-transparent but with HDR on they become fully opaque.
This might hint at the actual issue here being that when HDR is on the transparency or light accumulation is not being calculated correctly

HDR off + brightness

HDR on + brightness

Reproduce

1. Load the Atmosphere Sandcastle
2. Turn off Fog and Ground atmosphere just so they don't distract, these do not seem to be impacted
3. Move the camera to be looking at the horizon as shown in the pics above
4. Toggle HDR under the Sky Atmosphere tab
5. slide the Brightness Shift up higher to observe the "transparent vs opaque" oddity