Include cstddef for offsetof in the renderer

Older GCC (Debian 12's) doesn't pull it in through other headers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/renderer.cpp

@@ -1,6 +1,7 @@
 #include "renderer.h"
 #include <SDL3/SDL.h>
 #include <algorithm>
+#include <cstddef>
 #include <cstring>
 
 #define GL_GLEXT_PROTOTYPES