Fix sampling bug when SSAO is using half size

[clayjohn]

Fixes: #62881
Fixes: #66643

There are two pieces to this PR:

1. When downsampling 5 mip levels at once, the 5th mip level was showing some artifacts which lead to invalid samples in the SSAO shader. To fix it, I have removed the invalid averaging code from the 5th mipmap computation. This code path was only hit when using both SSAO and SSIL and only one of them is using half size
2. Change the half_screen_pixel_size to be based off the full screen size / 2. This was a difficult bug to track down, but essentially when using a screen size that could not be subdivided cleanly 4 times the half res samples would drift away from their intended positions. To fix that, we base our scaling off of the full viewport size.

This also cleans up two unreported bugs:

1. The SSAO shader was using the wrong sampler (default instead of mirror)
2. The depth linearization process was doubling the depth value. This could have led to SSAO improperly rejecting valid occlusion (i.e. appearing slightly lighter than it should).

Finally, on investigation it appears that SSIL was suffering from the same issues, it was just less obvious. So I made the changes to SSIL as well

Before

After

Before

After

[BastiaanOlij]

After discussing further with Clay, agreed this seems like a sound fix

[akien-mga]

Thanks!