Divide the do_paint() loop into several subloops. The activity of

going through the line and working out which bits need to be redrawn
is now in a separate loop from the subsequent activity of actually
going through and doing the redraws. This _should_ enable me to
tinker with the which-bits-to-redraw data in between the two, thus
fixing `font-overflow'. However, I thought it would be sensible to
break the work up into two commits so we can track bugs in the
restructuring separately from bugs introduced by the new feature.

Also added a couple more terminal test files.

git-svn-id: svn://svn.tartarus.org/sgt/putty@5001 cda61777-01e9-0310-a592-d414129be87e