texture() uses bilinear scaling; imageLoad() accesses the image directly.
The reason why texture() was used throughout Vulkan filters is that
back when they were written, they were targetting old Intel hardware,
which had a texel cache only for sampled images.
These days, GPUs have a generic cache that doesn't care what source it
gets populated with. Additionally, bypassing the sampling circuitry saves
us some performance.
Finally, all the old texture() code had an issue where unnormalized
coordinates were used, but an offset of 0.5 was not added, hence each
pixel ended up being interpolated. This fixes this.
