It's useful for extension scripts to be able to get the file offset
of symbols in non-addressable regions. One example of this is CHR
ROM data for an NES cartridge. However, we were getting the offset
by doing an address-to-offset mapping on the plugin side, which by
definition doesn't work for non-addressable memory.
So we now add the offset to PlSymbol objects for user labels and
address region pre-labels. The NES visualizer has been updated to
use the new field.
Also, fixed a bogus complaint about bank overruns for non-addressable
regions.
Added a visualizer for the CHR ROM pattern tables, and a semi-useful
visualizer for tile grids.
Also added a few chars in an 8x8 font that visualizers can use to
label things.
