Spindle speed overrides behavior tweak. New experimental laser dynamic power mode.

- Spindle speed overrides now update immediately if they are changed
while in a HOLD state. Previously, they would update after exiting the
HOLD, which isn’t correct.

- New experimental dynamic laser power mode that adjusts laser power
based on current machine speed. Enabled by uncommenting
LASER_CONSTANT_POWER_PER_RATE in config.h

  - It assumes the programmed rate is the intended power/rate for the
motion.
  - Feed rate overrides (FRO) do not effect the power/rate. Meaning
that spindle PWM will automatically lower with lower FRO and increase
with higher FRO to keep it the same.
  - Spindle speed overrides (SSO) will directly increase and decrease
the power/rate. So 150% SSO will increase the PWM output by 150% for
the same speed.
  - The combination of FRO and SSO behaviors should allow for subtle
and highly flexible tuning of how the laser cutter is operating in
real-time and during the job.

- Re-factored planner block rapid rate handling for the dynamic laser
power feature. Should have had no effect on how Grbl operates.

grbl/planner.h

@@ -45,6 +45,7 @@
 #define PL_COND_FLAG_SPINDLE_CCW       bit(5)
 #define PL_COND_FLAG_COOLANT_FLOOD     bit(6)
 #define PL_COND_FLAG_COOLANT_MIST      bit(7)
+#define PL_COND_MOTION_MASK    (PL_COND_FLAG_RAPID_MOTION|PL_COND_FLAG_SYSTEM_MOTION|PL_COND_FLAG_NO_FEED_OVERRIDE)
 #define PL_COND_ACCESSORY_MASK (PL_COND_FLAG_SPINDLE_CW|PL_COND_FLAG_SPINDLE_CCW|PL_COND_FLAG_COOLANT_FLOOD|PL_COND_FLAG_COOLANT_MIST)