Fixed recent DoF and tonemap bugs

[clayjohn]

Fixes: #61165 and fixes #61167

Both bugs introduced by #54585

The tonemapping bug comes from the ANGLE shader compiler being strictly compliant with the GLSL specification and refusing to subtract a float from an int. The solution is to just use floats.

The DoF bug is more subtle. #54585 allows some post-effects to run with a transparent background. However, DoF near overrides the alpha channel of the backbuffer to pass extra information from the horizontal pass to the vertical pass. This invalidates the alpha values the alpha channel of the buffer. The solution is to ignore alpha when writing out the vertical pass. This achieves two things:

1. the dark artifact is gone becuase the buffer maintains its alpha of 1
2. DoF will still work on a transparent VP, it just won't look totally correct as the blended alpha is ignored.

To properly implement DoF with a transparent Viewport we will need another buffer. Since this is something that we didn't support prior to 3.5 and have had very little demand to support, I do not think we should attempt a full implementation before 3.5. If there is strong user demand for DoF with a transparent viewport then we can consider adding it in the nect release.

[akien-mga]

Thanks!

[TokisanGames]

@clayjohn Thanks. Is it plausible this also fixed another coloring artifact I experienced at the same time?
https://twitter.com/TokisanGames/status/1526975579844538370

The background house is colored differently. Far DOF is disabled. This environment is using TimeOfDay, which uses a giant transparent sphere mesh for fog, and a sky shader, and it controls the directional light. We are using tonemapping.

[clayjohn]

@clayjohn Thanks. Is it plausible this also fixed another coloring artifact I experienced at the same time? https://twitter.com/TokisanGames/status/1526975579844538370

The background house is colored differently. Far DOF is disabled. This environment is using TimeOfDay, which uses a giant transparent sphere mesh for fog, and a sky shader, and it controls the directional light. We are using tonemapping.

@tinmanjuggernaut It's possible. The DoF bug came from DoF turning parts of the buffer transparent, so there was bleedthrough from whatever was behind. If you send me an MRP I can take a look.

[TokisanGames]

The second coloring artifact is indeed still present. I'll make a separate issue for it.
https://twitter.com/TokisanGames/status/1526975579844538370