PAM controls the position of the prime line/blob

[jornamon]

With this patch, PAM can control the position of the prime line/blob of RatOS. It uses the first layer coordinates given to the MESH_CONFIG command to find the best place for the prime line/blob. It places the prime feature where there is more room, and the y-axis direction is towards the PAM, so that the prime gets done as close as possible to the measured area to avoid crashes.

It only updates the RatOS macro variables accordingly and let the PRIME macros do their thing, so it should still work even under changes in those macros that don't involve changes in the relevant variables.

Two parameters control the behavior:

• pam_controls_prime_position -> Whether PAM controls the position of the prime line / blob.
• prime_offset -> Offset from the print area to the start position of the prime line / blob (both x and y)

It has a simple sanity check and seems to work well in my VC3-500, but if definitely can be much improved.