This brings the behavior of mode 47, 1047, and 1049 much closer to
xterm's behavior. I found that our prior implementation had many
deficiencies.
For example, we weren't properly copying the cursor state back to the
primary screen from the alternate screen for modes 47 and 1047. And we
weren't saving/restoring cursor state unconditionally for mode 1049 even
if we were already in the alternate screen.
These are weird, edgy behaviors that I don't think anyone expected
(evidence by there being no bug reports about them), but they are bugs
nontheless.
Many tests added.
c5e5d61438