vt: selection: take screen contents from uniscr if available

This preserves whatever was written even if we can't currently display the
given glyph.  Mouse paste won't corrupt any character of wcwidth() == 1
anymore.

Note that for now uniscr doesn't get allocated until something reads
/dev/vcsuN for that console, making this code dormant for most users.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>