Take FXAA samples from half-pixel coordinates to improve quality

[clayjohn]

Fixes: #64985

It looks like the code we use for FXAA is a simplified model based on: https://www.geeks3d.com/20110405/fxaa-fast-approximate-anti-aliasing-demo-glsl-opengl-test-radeon-geforce/3/

One optimization this code makes is to only sample the four corners instead of taking all 8 samples around the current pixel. To make that effective, we need to use linear sampling and offset the samples so they overlap all 8 pixel positions. This can be seen in the linked code, the sample positions are at half-pixel offsets. However, when we implemented the code we used full pixel offsets so we are only getting contributions from the four corner pixels which leads to a lot of missing data which create artifacts.

This change results in a smaller blur, so FXAA is less blurry, but also a reduction in artifacts so there are fewer random jaggies created by the FXAA. This results in both smoother edges and less blurring.

This change can be made in 3.x as well.

Before:

After:

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.6.