Shaders: allow default construction of MeshVisualizer.
Shaders: allow default construction of MeshVisualizer.

While it doesn't make any sense, it is more convenient than writing {}
when testing it.

MiUishadow/magnum,DerThorsten/magnum,ashimidashajia/magnum,MiUishadow/magnum,ashimidashajia/magnum,DerThorsten/magnum,DerThorsten/magnum,DerThorsten/magnum,MiUishadow/magnum,ashimidashajia/magnum,ashimidashajia/magnum,ashimidashajia/magnum,DerThorsten/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum