Fix crown gear preview mode in PartDesign Bodies

Previously, the `preview_mode` of the crown gear returned a compound of
the base and the cut-outs. This caused problems in PD::Bodies where a
single solid is requried.
The solution in this commit changes the preview_mode to only output the
base, not generating the cutout shapes at all. This is consistent with
the involute gears having "simple=true" and saves again 0.5 Seconds
processsing time on my system using defaults (15 teeth, 4 loft profiles).

In addition, "preview = false" is also speed up by first collecting all
cut-outs, and then passing them all at once to a single cut operation.
This reduced the cutting time from 3.0 Seconds to 2.2 Seconds here.
So preview now generats the shape immediately (0.0008s vs 0.5s) and the
actual crown is generated in 2.7s instead of 3.5s (again, using the
defaut parameters, measued via Python's time.perf_counter).

freecad/gears/features.py

@@ -422,6 +422,8 @@ class CrownGear(BaseGear):
         inner_circle.reverse()
         face = Part.Face([outer_circle, inner_circle])
         solid = face.extrude(App.Vector([0., 0., -fp.thickness.Value]))
+        if fp.preview_mode:
+            return solid
 
         # cutting obj
         alpha_w = np.deg2rad(fp.pressure_angle.Value)
@@ -443,17 +445,11 @@ class CrownGear(BaseGear):
         loft = makeLoft(polies, True)
         rot = App.Matrix()
         rot.rotateZ(2 * np.pi / t)
-        if fp.preview_mode:
-            cut_shapes = [solid]
-            for _ in range(t):
-                loft = loft.transformGeometry(rot)
-                cut_shapes.append(loft)
-            return Part.Compound(cut_shapes)
-        else:
-            for i in range(t):
-                loft = loft.transformGeometry(rot)
-                solid = solid.cut(loft)
-            return solid
+        cut_shapes = []
+        for _ in range(t):
+            loft = loft.transformGeometry(rot)
+            cut_shapes.append(loft)
+        return solid.cut(cut_shapes)
 
     def __getstate__(self):
         pass