Platform: undefine another conflicting macro.
Our own aliases for builtin types.

It seems like a bad idea, but it will:

 * Improve portability, as `Int` will be always 32bit.
 * Improve readability, as `std::int32_t` is just plain ugly and too
   complicated to write.
 * Improve consistency and reduce confusion, as it's not good to mix
   `int`, `std::int32_t`, `GLint`, `khronos_int_t` and whatnot in one
   codebase.
 * Possibly reduce compilation time, because including all ~35k lines
   worth of GL headers just for one GLfloat typedef is even worse than
   now forbidden #include <iostream> in headers.
Math: using new aliases for builtin types in whole Math namespace.
Using new type aliases in whole root Magnum namespace.
Trade: using new type aliases in whole Trade namespace.
Primitives: using new type aliases in whole Primitives namespace.
MeshTools: using new type aliases in whole MeshTools namespace.
Shaders: using new type aliases in whole Shaders namespace.
SceneGraph: using new type aliases in whole SceneGraph namespace.
Physics: using new type aliases in whole Physics namespace.
Text: using new type aliases in whole Text namespace.
Platform: using new type aliases in whole Platform namespace.
DebugTools: using new type aliases in whole DebugTools namespace.
Removed unneeded includes.
Moved OpenGL includes out from Magnum.h.

OpenGL includes are ~35k lines together and it is a waste of
compilation time to include them even if they are not needed at all
(e.g. whole SceneGraph and Physics libraries). Saves ~10s of compilation
time (6:46 before, now 6:35).