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/opengl/context.cpp

@@ -34,6 +34,9 @@ context::context(window const& win) {
         return;
     }
     m_valid = true;
+
+    // Set initial viewport to match window size
+    glViewport(0, 0, win.width(), win.height());
 }
 
 context::~context() {}
@@ -42,4 +45,8 @@ auto context::valid() const -> bool {
     return m_valid;
 }
 
+auto context::set_size(int width, int height) -> void {
+    glViewport(0, 0, width, height);
+}
+
 }