Implemented showing program extents

docs/todo.txt

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+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
+
+Render program extents
+    - show a blue bounding box for the program extents