Fix position shift with MBL

[thinkyhead]

Addressing #4266

When MBL is enabled and a position is set with G92 (or other functions —like gcode_T tool-change— that reorient the current position) the destination ends of the mesh-split lines are calculated at the wrong positions. As a result we see strange moves after a tool-change when MBL is enabled.

• Also note that MBL can actually handle up to 9 x 9 grids.
• Split out the mbl.active check so other line buffering is possible.
• Since mesh_buffer_line always uses destination, use it as the target
• Along that line, make mesh_buffer_line_to_destination instead
• Apply other optimizations to reduce code size (by 654 bytes)

[thinkyhead]

@epatel Note the main bug-fix here: We forgot to include the newly-inroduced position_shift[axis] when calculating the edges of the splits. So any code that modified position_shift would cause incorrect moves once MBL was enabled.

As a bonus, I realized that MBL can handle a slightly finer grid – up to 9 x 9 – because the x_splits/x_splits bit-masks use the grid index (0 - 7). That will help with the larger beds.

[epatel]

One thing. Removing the recursion is not good. I hope you test everything else to verify, my brain hurts trying to follow all changes.

Below is one case that will fail when doing like that. When moving "From" to "To" the first split will be on the X boundary (B) because that will be the first "if" statement that gets selected, then split A is lost which would have been found by the recursion.

[thinkyhead]

Makes sense!

[thinkyhead]

I realize the changes are many, and I didn't have them well laid out. So I went back and re-did the commits in order of obviousness, leaving off the dropped recursion.

• The first thing was just to find the pattern and do the most obvious reduction to bring the code size down a little bit.
• Then there was a little more redundancy in setting nz and ne for next end-point.
• Next, since it's being used like other move functions, just use destination (instead of x, y, z, e arguments). This puts off making a copy on the stack until needed.