fix(fonttools): scale Bezier subdivision to the pixel error

Flatten each quadratic with a segment count derived from
|p0 - 2*p1 + p2| in pixel space, targeting < 0.5 px error. The count
grows with the rendered font size. All-off-curve contours now use
midpoint quadratics instead of polygon edges.

vpsdk/fonttools/fonttools.vp

@@ -176,10 +176,18 @@ def _add_edge(x0: float, y0: float, x1: float, y1: float):
     _edge_count += 1
 
 def _quad(x0: float, y0: float, x1: float, y1: float, x2: float, y2: float):
+    # Maximum distance from the chord is |p0 - 2*p1 + p2| / 4.
+    # Choose n so the error per segment is below 0.5 px.
+    ddx: float = x0 - 2.0 * x1 + x2
+    ddy: float = y0 - 2.0 * y1 + y2
+    d: float = math.sqrtf(ddx * ddx + ddy * ddy)
+    n: int = int(math.floorf(math.sqrtf(d / 2.0))) + 1
+    if n < 8: n = 8
+    if n > 64: n = 64
     px: float = x0
     py: float = y0
-    for s in range(1, 9):
-        t: float = float(s) / 8.0
+    for s in range(1, n + 1):
+        t: float = float(s) / float(n)
         mt: float = 1.0 - t
         x: float = mt * mt * x0 + 2.0 * mt * t * x1 + t * t * x2
         y: float = mt * mt * y0 + 2.0 * mt * t * y1 + t * t * y2
@@ -194,10 +202,21 @@ def _build_contour(start: int, end: int):
             first_on = i
             break
     if first_on < 0:
-        for i in range(start, end + 1):
-            j: int = i + 1
-            if j > end: j = start
-            _add_edge(_px[i], _py[i], _px[j], _py[j])
+        sx: float = (_px[end] + _px[start]) / 2.0
+        sy: float = (_py[end] + _py[start]) / 2.0
+        cx: float = sx
+        cy: float = sy
+        i: int = start
+        while i <= end:
+            nx: int = i + 1
+            if nx > end: nx = start
+            ex: float = (_px[i] + _px[nx]) / 2.0
+            ey: float = (_py[i] + _py[nx]) / 2.0
+            _quad(cx, cy, _px[i], _py[i], ex, ey)
+            cx = ex
+            cy = ey
+            i += 1
+        _add_edge(cx, cy, sx, sy)
         return
     cx: float = _px[first_on]
     cy: float = _py[first_on]