Keep FWRETRACT values in terms of millimeters when using M200

for volumetric E units

Marlin/Marlin_main.cpp

@@ -137,7 +137,7 @@
 // M204 - Set default acceleration: S normal moves T filament only moves (M204 S3000 T7000) im mm/sec^2  also sets minimum segment time in ms (B20000) to prevent buffer underruns and M20 minimum feedrate
 // M205 -  advanced settings:  minimum travel speed S=while printing T=travel only,  B=minimum segment time X= maximum xy jerk, Z=maximum Z jerk, E=maximum E jerk
 // M206 - set additional homeing offset
-// M207 - set retract length S[positive mm] F[feedrate mm/sec] Z[additional zlift/hop]
+// M207 - set retract length S[positive mm] F[feedrate mm/sec] Z[additional zlift/hop], stays in mm regardless of M200 setting
 // M208 - set recover=unretract length S[positive mm surplus to the M207 S*] F[feedrate mm/sec]
 // M209 - S<1=true/0=false> enable automatic retract detect if the slicer did not support G10/11: every normal extrude-only move will be classified as retract depending on the direction.
 // M218 - set hotend offset (in mm): T<extruder_number> X<offset_on_X> Y<offset_on_Y>
@@ -1128,10 +1128,12 @@ void process_commands()
         destination[Y_AXIS]=current_position[Y_AXIS];
         destination[Z_AXIS]=current_position[Z_AXIS];
         current_position[Z_AXIS]+=retract_zlift;
-        destination[E_AXIS]=current_position[E_AXIS]+retract_length+retract_recover_length;
+        destination[E_AXIS]=current_position[E_AXIS]+(retract_length+retract_recover_length)/volumetric_multiplier[active_extruder]; 
+        float oldFeedrate = feedrate;
         feedrate=retract_recover_feedrate;
         retracted=false;
         prepare_move();
+        feedrate = oldFeedrate;
       }
       break;
       #endif //FWRETRACT