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.
Start of AbstractImporter rework - Trade namespace will contain simple
classes usable for exchanging data with other formats, importers will
not depend on any OpenGL functionality, they will just provide access to
plain data.
Each importer must now specify whether it can open files or streams in
features() and implement one or both open() functions. Default
implementation of open() functions now prints message on error output
and returns false.
SizeTraits class provides suitable types for given data size at compile
time, SizeBasedCall can call suitable templated overload based on given
data size at runtime.
Also added meta classes Pow and Log for computing powers and logarithms
at compile time, usable mainly in conjunction with SizeTraits. Their
implementation is checked at compile-time using static_cast().