[qu2cu] Copy a function from cu2qu, to make Cython happy

Lib/fontTools/qu2cu/qu2cu.py

@@ -23,7 +23,6 @@ except ImportError:
     from fontTools.misc import cython
 
 from fontTools.misc.bezierTools import splitCubicAtTC
-from fontTools.cu2qu.cu2qu import cubic_farthest_fit_inside
 
 
 __all__ = ["quadratic_to_curves"]
@@ -37,6 +36,49 @@ else:
     COMPILED = False
 
 
+# Copied from cu2qu
+#@cython.cfunc
+@cython.returns(cython.int)
+@cython.locals(
+    tolerance=cython.double,
+    p0=cython.complex,
+    p1=cython.complex,
+    p2=cython.complex,
+    p3=cython.complex,
+)
+@cython.locals(mid=cython.complex, deriv3=cython.complex)
+def cubic_farthest_fit_inside(p0, p1, p2, p3, tolerance):
+    """Check if a cubic Bezier lies within a given distance of the origin.
+
+    "Origin" means *the* origin (0,0), not the start of the curve. Note that no
+    checks are made on the start and end positions of the curve; this function
+    only checks the inside of the curve.
+
+    Args:
+        p0 (complex): Start point of curve.
+        p1 (complex): First handle of curve.
+        p2 (complex): Second handle of curve.
+        p3 (complex): End point of curve.
+        tolerance (double): Distance from origin.
+
+    Returns:
+        bool: True if the cubic Bezier ``p`` entirely lies within a distance
+        ``tolerance`` of the origin, False otherwise.
+    """
+    # First check p2 then p1, as p2 has higher error early on.
+    if abs(p2) <= tolerance and abs(p1) <= tolerance:
+        return True
+
+    # Split.
+    mid = (p0 + 3 * (p1 + p2) + p3) * 0.125
+    if abs(mid) > tolerance:
+        return False
+    deriv3 = (p3 + p2 - p1 - p0) * 0.125
+    return cubic_farthest_fit_inside(
+        p0, (p0 + p1) * 0.5, mid - deriv3, mid, tolerance
+    ) and cubic_farthest_fit_inside(mid, mid + deriv3, (p2 + p3) * 0.5, p3, tolerance)
+
+
 @cython.locals(_1_3=cython.double, _2_3=cython.double)
 @cython.locals(
     p0=cython.complex,