Vector4 doesn't set W component to one by default anymore, this is now
handled by Point*D itself. This finally allows creating of 2D primitives
and 2D position vectors without messing explicitly with Z = 1.
All classes which should use Point instead of Vector were updated to use
Point instead.
Magnum.h now doesn't include anything except OpenGL headers, thus
changes in Math library don't trigger recompilation of everything, but
only of things really depending on it.
Math constants moved to separate file for similar reasons, de-inlined
some functions to remove the need for some #includes.