This fixes a conflicting definition of GLsizeiptr and GLintptr when
Magnum is combined with qopengl.h from Qt on Windows. Khronos has it as
`signed long int`, whereas qopengl.h as ptrdiff_t, which is just `int`.
The actual WebGL 2 distinction was done as a part of previous WebGL
cleanup. It compiles, but is completely broken, as Emscripten has some
missing bits to go through at least Context class creation.
Currently just regenerated the stock headers using flextGL, reducing
line count a bit. No dynamic extension loading or function pointers yet,
trying to make as little changes as possible.