Fixes #9

The tfmofm.ltx is special since this specific issues can't be tested in
plain (cm math fonts which trigger the issue are dumped in the format,
so the ttf version never get loaded in plain).

We now detect tfm fonts much earlier and force a file based lookup,
thus bypassing |names.resolve()| completely.

So, now |\font\tenrm=cmr10| is exactly the same as |\font\tenrm=file:cmr10|
as long as there is a tfm/ofm file named cmr10.

Code scrapped from ConTeXt, AFAIK, no font implements OpenType optical
bounds (on of Linux Libertine font has traces of it), but I plan to use
it in my fonts. The committed feature file is prepared for Pagella, we
can later have font specific feature files like this.

With the previous commit, if we don't found a font with the requested
optical size, we return nothing, though we might have a matching font
name.

Now we store all matching names with no matching optical size and return
the first one, then we return the closest match (i.e. if the requested
size is 15pt and we have 10pt and 5pt match, we return the 10pt one).

We now simply loop over the whole data.mappings table, so loading font
by family, fullname, psname etc is all the same,  but it is a bit slower
now (~0.25s with syatemfonts.tex).

Now, if we didn't find a matching family name, we then try fullname.

Some fonts give only design size with no range, in this case we now will
assume range top == range bottom == design size.