fix full colour glyph fetch bug.
make colour 0 transparent in full-colour mode.
begin writing test programme to show thumbnail of screen.
clean up default target, and add thumbnail.prg to target list.
slow down writing to avoid mis-writes.
set palette. clear $FF glyphs correctly.
try to fix advancing between glyph rows when drawing thumbnail.
more work on thumbnail drawing.  still address calculation errors
on Y coordinate.