Ensure post processing happens when adjustments are enabled in the Compatibility renderer

[clayjohn]

Fixes: #92853

This issue was caused by how we manage our post-processing effects. Right now we only do post processing when the internal buffers are allocated. We check if the internal FBO exists (i.e. fbo != 0) as our condition for doing post effects.

The internal FBO is only ever allocated when glow or scaling are used. If neither of those are used then no post processing happens. Accordingly, the problem here was that adjustments didn't properly request for the internal FBO to get allocated.

Previously it was requested by Glow each frame. If Glow was enabled or disabled the existing buffers would get thrown out and re-allocated. I have adjusted this logic to simply request that the internal buffer be created. We don't need to free the buffers at run time. They should be allocated if needed, and then kept around to avoid performance penalties if effects are adjusted at run time.

I have fixed a few related issues too:

1. The old function could leak memory. If the MSAA setting changed, the internal buffers would get re-allocated without freeing the old versions. This would leak them. To fix that, it now checks that the buffers have been freed before creating them.
2. The documentation said that adjustments aren't supported. I removed that note.

[Calinou]

Tested locally, it works as expected. Code looks good to me.

[akien-mga]

Thanks!