TriMesh cannot be default constructed

[richardeakin]

Sometimes it is convenient to have a TriMesh as a member variable on your App class or something similar, however this isn't currently possible because it has no default constructor. Could we not just add a default Format() like so, to get the default constructor?

TriMesh( const Format &format = Format() );

[richardeakin]

Actually it seems like if a TriMesh were to be default constructed on the stack, it would have the same layout as if heap allocated by TriMesh::create() with no arguments, like:

 TriMesh( const TriMesh::Format &format = TriMesh::Format().positions().normals().texCoords() );

[richardeakin]

Fixed with #874