[FR] G29 J2 should use bilinear correction

Description

Currently, G29 J2 appears to fit a plane, as after doing it, the surface at the probed points is not exactly zero. However, the only reason to use 4-point (or more) leveling rather than 3-point is that the bed isn't, in fact, a plane. It would be great if G29 J2 could instead be used to apply an overall bilinear correction (or meshed for >2) to the loaded mesh.

Steps to Reproduce

1. Run G29 J2.
2. The height of the probed points should be exactly zero, like they are if you use a 2x2 UBL mesh and probe it with G29 P1.

Additional Information

I used to use bilinear leveling, but the shape of my bed deviates significantly (by ~0.5mm) from a bilinear shape. To correct for this, I want to use UBL. I've probed a 7x7 grid of my bed. However, depending on exact temperature etc, the overall height of the bed and the length of the nozzle changes. For that reason, I want to use a G29 J2 to apply an overall bilinear correction to the previously probed mesh. When I do this now, it results in a significant (~0.3mm) offset towards the edges of the grid. I think this is because it fits a plane but I already know a plane is not a good description of the shape of the bed.

From glancing at the code for G29 J in ubl_G29.cpp, it appears that it doesn't even approximate the difference between the loaded mesh and the actual probed points with a plane, it turns off the leveling data, fits a plane to the probed points, and then rotates the mesh by that plane.

If we have to fit a plane, I believe a better way of doing it would be to fit the plane to the points including correction. But using bilinear should be both easier and better.

Start in ubl_G29.cpp at the G29 J handling and compare its current correction behavior with the 2x2 UBL mesh behavior described in the issue. Check how the loaded mesh and probed points are used, then verify with the reproduction steps that the probed points reach zero and the existing mesh shape is preserved.