remove unneeded includes in public headers

Replace with forward declarations if needed and includes in .cpp that
now need them.
The idea here is to have our headers have the least amount of impact as
possible on our clients (e.g. compilation time).

samples/sample_normal_map.cpp

@@ -28,6 +28,7 @@
 #include <filament/Material.h>
 #include <filament/MaterialInstance.h>
 #include <filament/RenderableManager.h>
+#include <filament/TextureSampler.h>
 #include <filament/TransformManager.h>
 #include <filament/Scene.h>
 #include <filament/Texture.h>