Clear screen in render if we failed to load a render function

Better than flickering uninitialised data

main.cpp

@@ -60,7 +60,12 @@ void init() {
 }
 
 void render(uint32_t time) {
-    if(!has_render) return;
+    if(!has_render) {
+        blit::screen.pen = {0, 0, 0};
+        blit::screen.clear();
+        return;
+    }
+
     uint32_t ms_start = now();
     lua_getglobal(L, "render");
     lua_pushnumber(L, time);