REVERTED: Removed the need for rlMatrix

Now rlgl uses the `Matrix` type, just make sure it has been previously defined somewhere... I don't like this approach but it's probably the easier one for the users... still looking for a better solution... maybe using something like
`#define MATRIX_TYPE`, so it can be checked in other modules.

examples/models/models_skybox.c

@@ -11,6 +11,7 @@
 
 #include "raylib.h"
 #include "rlgl.h"
+#include "raymath.h"
 
 #if defined(PLATFORM_DESKTOP)
     #define GLSL_VERSION            330