artist: only render dirty scanlines on the display surface

artist: only render dirty scanlines on the display surface

The framebuffer_update_display() function returns the dirty scanlines that were
touched since the last display update, however artist_update_display() always calls
dpy_gfx_update() with start and end scanlines of 0 and s->height causing the
entire display surface to be rendered on every update.

Update artist_update_display() so that dpy_gfx_update() only renders the dirty
scanlines on the display surface, bypassing the display surface rendering
completely if framebuffer_update_display() indicates no changes occurred.

This noticeably improves boot performance when the framebuffer is enabled on my
rather modest laptop here, including making the GTK UI usable.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20220504153708.10352-4-mark.cave-ayland@ilande.co.uk>
Reviewed-by: Helge Deller <deller@gmx.de>
Reviewed-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>