Updated FindCorrade.cmake and FindNodeJs.cmake from Corrade repository.
Don't use `constexpr` in Timeline.

There is no point in using this class at compile-time. Ever. Moreover
clang called through Emscripten complains that it cannot be constexpr.
Clang called normally doesn't complain, don't know why.
Method chaining in Timeline.
Emscripten: ported FindOpenGLES2.cmake and FindSDL2.cmake.

SDL headers are now included relatively.
Updated NaCl-related documentation and stuff.
MeshTools: don't create reference from rvalue.

Spotted by Emscripten/Clang, not found when running clang normally.
Platform: moved Sdl2Application event loop into separate function.

Allowing it to be called perodically from some callback.
Platform::Sdl2Application: just wait for next event.

No need to loop over when SDL has function designed to do this.
Force static build if building with NaCl's newlib.
Force OpenGL ES 2.0 target if building for Emscripten.