Various documentation fixes and updates.

{} in Matrix constructor caused Doxygen to forget all functions after
it.
Platform: rename Sdl2Application::Configuration::Flags to WindowFlags.

Flags etc. will be used for context flags in the future and this would
result in naming collision.
SceneGraph: template implementation file for Drawable class.

The class has virtual member, so it's good to have it in one place and
not in thousand five ones.
SceneGraph: documentation update for feature classes.
Documented texture formats disallowed in *Texture::setStorage().
Removed forgotten call from Mesh test.

No change in test results.
Platform: don't store useless variable on Emscripten.
Fix compilation on Emscripten.
Added some TODOs.
Documented internal formats unsupported in 3D textures.
Properly error out when using unsupported header.