New mathfwd.h header

math/mathwfd.h forward declares all {mat|vec}{2|3|4}<> classes,
which allows us to remove their respective #include in a lot of
our public headers.
Our math headers are full of templates, so this should help build times
a bit.

Also we want to keep the public headers as minimalist as possible.

android/filament-android/src/main/cpp/IndirectLight.cpp

@@ -20,6 +20,7 @@
 #include <filament/Texture.h>
 #include <common/NioUtils.h>
 #include <common/CallbackUtils.h>
+#include <math/mat4.h>
 
 using namespace filament;