Added width/height tracking to the window class via a GLFW
resize callback. The callback stores the current size and
forwards it to a user-provided std::function.
Added context::set_size() to update the GL viewport when
the window is resized.
Changed scene::render() to accept (int width, int height)
parameters so scenes can compute the correct aspect ratio
for their projection matrix instead of hardcoding 1280/720.
Fixed parameter shadowing in resize_callback_impl
(glfw_window instead of window).
portersky
a4ef4adfc7