Minor documentation fixes.
Math: added Vector::isZero().
Added todo for the time when GCC 4.6 support is dropped.
std::decay in fact does the thing we need.

Exactly this approach is used in e.g. std::make_optional, as seen on
cppreference.com. Improves 68638969f3bbcd2f7f8a9a17619c1b1680146229.
Doc: reworded and improved compilation speedup page.
Doc: updated portability page.
Failed attempt to resolve Doxygen file conflicts.

Found even more stuff broken -- copied contents of
Corrade::Utility::Directory::Flags into Magnum::Context::Flags (WTF!),
asterisks in code listings... IS IT SO HARD TO FIX ONE THING AND DON'T
BREAK THOUSAND OTHERS, DAMMIT??
Math: improve integer/float Vector multiplication and division.

Operators that are part of Vector are operating only with the same type
as Vector itself, operators for multiplying/dividing integral vectors
with floating-point numbers and vectors are now out-of-class and enabled
only for integer vectors. It allows better control (e.g. multiplying
integer and floating-point vector will _always_ result in floating-point
one). Thoroughly tested integer/FP operations and also reworked and
tested operator and funciton reimplementations in subclasses, both for
value correctness and result type correctness.
Math: removed integer/float RectangularMatrix multiplication and division.

We would need to duplicate all the functionality found in Vector, which
I don't think is needed at all. If anyone needs to do this, it is
possible to "linearize" the matrix into long vector and do the
operations on it.

Also updated subclass operator implementation, added tests for it, both
for proper returned value and proper result type.
Math: test also Matrix subclass function implementations.

Similarly to what is already done for Vector and RectangularMatrix.
Math: no need to explicitly overload operator=().

It is created automatically.
Math: fix and test RectangularMatrix*Vector operation.

How did I forget to test that?
Math: added bitwise operations for integer Vector.
Use `size >> mipLevel` to compute image size at given mip level.

This is so awesome trick.
Updated matrix/vector documentation.
Gaah.
Shapes: preparation for non-boolean collision queries.
Doc: added Khronos OpenGL ES presentation to Best Practices.
Merge branch 'master' into compatibility

Conflicts:
	src/AbstractTexture.cpp
GCC 4.5 compatibility: nullptr-related issues.