[FR] Start the Bed Mesh Validation Pattern with one of the Full Circle pads instead of the front left corner

[Thinkersbluff]

Is your feature request related to a problem? Please describe.

The pattern currently starts at 0,0. (after priming)
There, it tries to draw 1/4 of a circle, which involves a lot of quick short arcs and Z-Lift movements.
That first pad is often badly drawn - perhaps because the nozzle has a tendency to pull the end of each line back up with each Z-Lift movement, and those threads can be knocked by the head on subsequent passes.

Marlin's G26.cpp supports starting the pattern at any of the mesh points, it need not start at 0,0.

If the gcode string in MeshValidationHandler.cpp (line 72) is modified to use X117,Y117 instead of X0,Y0, for instance, then the pattern begins with one of the full circle pads in the center of the bed (on the CR6-SE).

It is my experience that when I make that change, the front left corner pad is now always drawn as well as all the other corners, instead of failing.

This change only requires that one edit to implement, and solves the current problem of the front left pad often being torn or knocked-off.

Are you looking for hardware support?

No.
This change would work with all configurations of CR6 SE and MAX, whether stock or modified.

Describe the feature you want

When I run the Bed Mesh Validation pattern, I want the pattern to begin by drawing one of the full circle pads, not by trying to draw the 1/4 circle at 0,0.

My experience is that the current firmware rarely if ever draws that front left 1/4 circle pad correctly, suggesting that there is always something wrong with that mesh point. In fact, however, if the pattern is started on one of the full circle pads first, on my system, it is able to draw that front left 1/4 circle cleanly every time, with the same bed mesh.