… the

texture filtering hardware more effectively.

This new shader performs Gaussian *resampling* instead of regular convolution.
It samples in between texels to reduce the number of taps.

The speed is about the same as the existing technique. It is nevertheless the
fastest blur method that I could come up with. In particular, it exceeds the
performance of the Kawase and dual Kawase blur techniques. The speed comes from
working at lower resolution and incurring fewer downsampling and upsampling
passes. As is often the case, ALU performance does not really seem to be the
limiting factor; it's mostly memory bandwidth, which is why downsampling is so
important. Further improvements should come from not doing the downsampling at
all and instead rendering the blurred content at low resolution to begin with.

Closes #2821.