Fix cogl_texture_get_data when an intermediate buffer is used

When an intermediate buffer is used for downloading texture data it
was using the wrong byte length for a row so the copy back to the
user's buffer would fail.

cogl/cogl-texture.c

@@ -1232,7 +1232,7 @@ cogl_texture_get_data (CoglHandle       handle,
         {
           src = new_bmp.data + y * new_bmp.rowstride;
           dst = data + y * rowstride;
-          memcpy (dst, src, new_bmp.width);
+          memcpy (dst, src, new_bmp.width * bpp);
         }
 
       /* Free converted data */