Fix tonemapper shader to correctly apply alpha channel

[Chaosus]

Changed a tone map shader to use an alpha channel from the input texture.

This should fix a bug with transparent_bg in viewports:

Fix #40651

Bugsquad edit: master version of #54585.

[BastiaanOlij]

This looks good to me and makes sense. I think there are other effects that may require a similar fix. I do know that DOF (I think) uses the alpha channel for weights and shouldn't overwrite the alpha of the end result.

[Chaosus]

@BastiaanOlij Yeah, I didn't touch other effects since I usually don't like to touch the things I've little to know ^^ to prevent accidental breakage.

[akien-mga]

Thanks!