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`.
It is superseded by core functionality. The only annoyance is that you
need to use TextureFormat::SRGB in ES2 and TextureFormat::SRGB8 in ES3,
but that's with many other formats anyway. Also apparently the unsized
format is still allowed in core desktop GL, which is a shame.
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.