Added fit, fixed the layer -1 bug finally, refactored here and there

src/window.h

@@ -5,6 +5,9 @@
 #define WINDOW_TITLE "Anm2Ed"
 #define WINDOW_TITLE_FORMAT "Anm2Ed ({})"
 #define WINDOW_FLAGS SDL_WINDOW_RESIZABLE | SDL_WINDOW_OPENGL
+#define WINDOW_TEST_MODE_FLAGS WINDOW_FLAGS | SDL_WINDOW_HIDDEN
+
+static const ivec2 WINDOW_TEST_MODE_SIZE = {1, 1};
 
 void window_title_from_path_set(SDL_Window* self, const std::string& path);
 bool window_color_from_position_get(SDL_Window* self, vec2 position, vec4* color);