Use the geometric center for shape first vertex

instead of the bounding box center.
The bounding box center can be on an edge (consider a right triangle).
Having the first vertex on an edge can make outline computation wrong by
failing to determine if an edge normal points inward or outward. The
geometric center being stricly inside the shape and not on an edge,
using it instead makes outline computation correct.

src/SFML/Graphics/Shape.cpp

@@ -220,9 +220,8 @@ void Shape::update()
     m_vertices[0]  = m_vertices[1]; // so that the result of getBounds() is correct
     m_insideBounds = m_vertices.getBounds();
 
-    // Compute the center and make it the first vertex
-    m_vertices[0].position.x = m_insideBounds.left + m_insideBounds.width / 2;
-    m_vertices[0].position.y = m_insideBounds.top + m_insideBounds.height / 2;
+    // Compute the geometric center and make it the first vertex
+    m_vertices[0].position = getGeometricCenter();
 
     // Color
     updateFillColors();