When using the Low or Medium settings on the Glare node, the generated
glare has an offset toward the lower left corner of the image. That's
because the shader erroneously has logic that assumes non-normalized
coordinates, so its offsets are in the pixel space of the original
image. To fix this, we just use a fixed 0.5 offset in the output pixel
space to evaluate at the center of pixels.
In the future, we might look into using a more advanced downsampling
scheme to avoid temporal instability, which is a new requirement due to
the viewport compositor.
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