G26's default behaviour ought to be the entire mesh (#6726)

Adding the capability to specify ahead of time how much of the validation pattern to print made it so by default G26 only did one circle and no connecting lines.

It is more natural for the unsophisticated user to just do the entire mesh (bed).   We default the repetition count to GRID_MAX_POINTS+1 to insure we get every last one of them!

Marlin/G26_Mesh_Validation_Tool.cpp

@@ -734,7 +734,7 @@
       random_deviation = code_has_value() ? code_value_float() : 50.0;
     }
 
-    g26_repeats = code_seen('R') ? (code_has_value() ? code_value_int() : 999) : 1;
+    g26_repeats = code_seen('R') ? (code_has_value() ? code_value_int() : GRID_MAX_POINTS+1) : GRID_MAX_POINTS+1;
     if (g26_repeats < 1) {
       SERIAL_PROTOCOLLNPGM("?(R)epeat value not plausible; must be at least 1.");
       return UBL_ERR;