It fixes#5046.
We just generate a mesh for the pattern rectangle where the color of each vertex is computed from the function.
Since the mesh is generated in the worker we don't really take into account the current transform when it's drawn.
That being said, there are maybe some possible improvements in using directly the gpu for the shading creation
which could then take into account the current transform, but it could only work with ps function we can convert
ino wgsl language and simple enough color spaces (gray and rgb).
On the worker-thread only the static `write` methods are actually used, and on the main-thread only class instances are being created.
Hence this, after PR 20197, leads to a bunch of dead code in both of the *built* `pdf.mjs` and `pdf.worker.js` files.
This patch reduces the size of the `gulp mozcentral` output by `21 419` bytes, i.e. `21` kilo-bytes, which I believe is way too large of a saving to not do this.
(I can't even remember the last time we managed to reduce build-size this much with a single patch.)
