Frame: guard sliver-producing refinement splits

src/frame.h

@@ -31,6 +31,9 @@ typedef struct {
   double jacobian_minimum;
   double min_edge_pixels;
   double min_area_pixels2;
+  /* Normalized image-plane quality, 1 for equilateral and 0 for degenerate.
+   * A non-positive value disables quality-driven red refinement. */
+  double min_triangle_quality;
 } RefinementConfig;
 
 typedef enum {