This signal is emitted when a significant area of the image being loaded has been updated.
Normally it means that a complete scanline has been read in, but it could be a different area as well.
Applications can use this signal to know when to repaint areas of an image that is being loaded.
X offset of upper-left corner of the updated area.
Y offset of upper-left corner of the updated area.
Width of updated area.
Height of updated area.