Constexpr

I added constexpr in some places, where the changes were light enough:
* I changed static const integer and integer arrays to static constexpr when the definition of the variables was not separated from the declaration.
* Point and Size are now literal types (see http://en.cppreference.com/w/cpp/concept/LiteralType) and can be used in constexpr expressions.

Since constexpr implies inline, it is not possible to define constexpr variables or functions in the .cpp file, except static constexpr variables defined at function scope. Therefore, the only variables and functions I changed to constexpr were already defined in .h or .inl files.