-PCJr, Tandy: implement vertical retrace interrupt,
-PCJr, CGA: do line-by-line video emulation,
-PCJr: support on-screen change of color modes 4medium to 16low (used by Ghostbusters booter),
-All machines: only update the video timing when needed (Jungle Hunt, others that synchronize to the video screen might profit), only resize the output window when needed, start up the video more quickly at the beginning.
Imported-from: https://svn.code.sf.net/p/dosbox/code-0/dosbox/trunk@3572