Move MeshReader into filamesh namespace.

This class will not extend well to glTF so this cordons it off into the
filamesh namespace, which is an already-existing namespace that we use
for the things related to the filamesh file format.

For glTF we might create a new library with its own MeshReader so
this will mitigate confusion.

samples/sample_normal_map.cpp

@@ -46,6 +46,7 @@
 
 using namespace math;
 using namespace filament;
+using namespace filamesh;
 using namespace filamat;
 using namespace utils;