only restore background when dispose is 3

Signed-off-by: Cocoa <i@uwucocoa.moe>

Author: cocoa-xu

stb_image.h

@@ -6815,6 +6815,9 @@ static stbi_uc *stbi__gif_load_next(stbi__context *s, stbi__gif *g, int *comp, i
                memcpy( &g->out[pi * 4], &two_back[pi * 4], 4 );
             }
          }
+
+         // background is what out is after the undoing of the previou frame;
+         memcpy( g->background, g->out, 4 * g->w * g->h );
       } else if (dispose == 2) {
          // restore what was changed last frame to background before that frame;
          for (pi = 0; pi < pcount; ++pi) {
@@ -6828,9 +6831,6 @@ static stbi_uc *stbi__gif_load_next(stbi__context *s, stbi__gif *g, int *comp, i
          // 1: do not dispose
          // 0:  not specified.
       }
-
-      // background is what out is after the undoing of the previou frame;
-      memcpy( g->background, g->out, 4 * g->w * g->h );
    }
 
    // clear my history;