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).

scenes/sphere.cpp

@@ -25,14 +25,14 @@ auto sphere::init() -> bool {
 
 auto sphere::update(float) -> void {}
 
-auto sphere::render() -> void {
+auto sphere::render(int width, int height) -> void {
     auto now = std::chrono::steady_clock::now();
     auto elapsed = std::chrono::duration<float>(now - m_start).count();
 
     glClearColor(0.15f, 0.15f, 0.2f, 1.0f);
     glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
-    auto aspect = 1280.0f / 720.0f;
+    auto aspect = float(width) / float(height);
     auto proj = glm::perspective(glm::radians(45.0f), aspect, 0.1f, 100.0f);
     auto view = glm::translate(glm::mat4{1.0f}, glm::vec3{0.0f, 0.0f, -4.0f});
     auto model = glm::rotate(glm::mat4{1.0f}, elapsed, glm::vec3{1.0f, 0.3f, 0.2f});