Fix AgX tonemapping using old approach in Compatibility rendering method

[Calinou]

• Follow-up to Add AgX tonemapper option to Environment #87260.

Its implementation now matches the one used in RenderingDevice-based rendering methods.

Not cherry-pickable to 4.3, as AgX is only in 4.4.

[allenwp]

This new code matches the rendering device shader. Results look as expected to me. AgX (and ACES for that matter) is not designed to work gamma space input, so I expect results to not necessarily look "good" in all cases. Here are some comparisons using Compatibility:

master AgX	this PR AgX	ACES

[Calinou]

AgX (and ACES for that matter) is not designed to work gamma space input, so I expect results to not necessarily look "good" in all cases.

I've empirically found I can get a better result by decreasing the Tonemap Exposure property to a value around 0.5, but it depends on the scene.

[akien-mga]

Thanks!