(graphics): mesh normals aren't computed each time

Description

Compute normals each time a mesh is updated (if mesh has no normals). Before it was only computed if mesh needs a reallocation (number of vertices greater than the previous mesh).

Closes #653 (closed)

How to test it?

Load several times a mesh without normals (probeMesh.vtk for ex.) in a scene with Ogre's SMesh adaptor.