Heightmap probing:
    1. define the grid:
        - from: x, y
        - to: x, y
        - step every: 5mm?
        - clearance height: Z1.5
        - start probing at: Z0.5
        - max negative z: Z-0.5 (when to fail probing)
        - z final safety height: Z15

        - from and to can be filled from current project bounding box

    2. probing
        - machine moves at grid start position (x, y)
            G0X0Y0
        - gets down for initial probing
            G38.2 Z-65 F100
        - step back a bit (1mm, relative)
            G91 G0 Z1
        - seek again with lower feed rate
            G38.2 Z-5 F5
        - after first probe ever, this Z becomes reference 0
            G10 L20 P0 Z0

        - all subsequent probes will be relative to this
        - for each probing location:
            - G0 Z"clearance height"
            - G0 XxxYyyy (next point location)
            - G0 Z"start probing at"
            - G38.2 Z-5 F5
            - store Z offset and place it in the heightmap

    3. modify loaded program with heightmap data
        - foreach line in program
            - if line contains a Z coordinate, update the Z according to the X and Y
            - if line does not contain a Z coordinate, add a Z coordinate according to the X and Y

Edge finding
    - prerequisites
        - which edge to find? -X/+X/-Y/+Y
        - bit diameter used
        - put the  

Corner finding
    - which corner are we probing: up left, up right, bottom left, bottom right
    - need to know the bit diameter
    - use G38.2 to probe X edge and set that as current X - diam/2


DONE - Show program extents

DONE - Render program extents
DONE    - show a bounding box for the program extents
