Fixes: #74709

The blurriness at the pole and seam comes from the shader reading from a higher mipmap automatically. Mipmap detection works by measuring the size of the derivative of the UV. I.e. the further away the UV coordinate is from its neighbour, the higher level we can read from. This assumption breaks with panorama coordinates as there is a huge jump on the seam (from 1 to 0) leading to a very high mipmap being used for the pixels on the seam.

PanoramaSkyMaterial never uses mipmaps, but it has the option to enable/disable filtering. We could add the option in the future, but no one has asked for it, so its best to just keep things simple and continue to always use linear