feat: add window resize support

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).

cbt/scene.cpp

@@ -8,6 +8,6 @@ auto scene::init() -> bool {
 
 auto scene::update(float) -> void {}
 
-auto scene::render() -> void {}
+auto scene::render(int, int) -> void {}
 
 }