Initialize the flood fill's dirty rectangle at its declaration

GCC cannot see that it is only read on a path where flood_region() wrote it,
and warns under -Wmaybe-uninitialized.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/draw.c

@@ -359,7 +359,9 @@ akerr_ErrorContext AKERR_NOIGNORE *akgl_draw_flood_fill(akgl_RenderBackend *self
     SDL_Surface *target = NULL;
     SDL_Surface *rgba = NULL;
     SDL_Texture *patch = NULL;
-    SDL_Rect dirty;
+    // Only written by a successful flood_region(), and only read after one, but
+    // the paths in between are far enough apart that the compiler cannot see it.
+    SDL_Rect dirty = { 0, 0, 0, 0 };
     SDL_FRect src;
     SDL_FRect dest;
     uint32_t *pixels = NULL;