Use simplified Z correction in G29 when possible

[thinkyhead]

For linear/planar bed leveling only.

• If XY hasn't changed since the last point was probed, then it's possible to use the last probe point to determine the "true" Z position. If the bed is flat (not warped) and the probe is accurate then this will produce the most accurate Z position possible.

An additional refinement would be to preserve the probed point array and compare the leveled Z at all probed points to the actual measured Z at all points, adding up and taking the average of the differences.

• Add conditionals for kinematics and leveling types
• Rename variables to better reflect their role in nonlinear leveling

[lolivet]

can i use this on my Robo 3D R1?

[thinkyhead]

@lolivet Sure, it's just Marlin, but attempts to correct the Z position at the end of G29.

[lolivet]

tanks bro. works perfectly