Remove redundant Z rescaling in multiview VoxelGI reconstruct_position

[luzader]

Fix for #116409
This is a draft because it works on the MRP, and "almost" works on a real XR headset.

After 4.3-dev6, the * 2.0 - 1.0 in the shader would seem to be double-correcting: set_depth_correction in gi.cpp should already be accounting for this.

MRP on Linux (MobileVR)

Fully fixes the issue, even if the camera is at arbitrary positions and angles.

MRP on Windows 11 (Bigscreen Beyond 2)

The scene actually has lighting now, but the lighting projection is distorted. I'm not sure how to describe the distortion, but (as you might expect) it varies with camera position.

Question

Is this flip_y really meant to be hardcoded to false, or should it be parameterized to use render_data->scene_data->flip_y, as it is in some other places in the codebase?

godot/servers/rendering/renderer_rd/environment/gi.cpp

Line 3871 in 8db94f7

correction.set_depth_correction(false);

[luzader]

Tried parameterizing the y_flip to use the scene_data flip_y, it didn't change the behavior. There's some other problem connected to voxel GI sampling that I haven't found.

[BastiaanOlij]

This is correct however it's only part of the solution, I did some more work in making GI work properly for stereo rendering here: #117303

[BastiaanOlij]

This fix was included in #117619 and should be resolved. Closing this PR.