Net: rewrite the drawing code for better scalability.

Previously, the network, grid edges and barriers were all drawn with
fixed line thickness of one pixel, or three pixels in the case of
wires (one central one in a lit-up colour and a black border pixel on
each side). This worked badly on high-DPI displays, or in any other
environment where you expand the window noticeably.

I've tried a few times before to fix this, but the problem was always
that the Net drawing code was complicated and confusing, because Net
was one of the founding puzzles in this collection and its drawing
code predated a lot of the sensible organisation and best-practice
doctrines I've come up with since then and used in the later puzzles.
(The best example of this was the way that redrawing a grid square
always redrew _all_ the surrounding borders, which is very confusing
in the light of my later practice of dividing up the grid into
disjoint regions that are always redrawn with clipping.) It was hard
to make any change to the Net graphics with the code in that state; I
tried several times and decided I couldn't sensibly make it work
without throwing it all away and rewriting from scratch, which I was
always reluctant to do.

But not any more! Since Ian sent some patches to improve the graphics
scaling in Tracks, and since Net was at the top of my personal
wishlist of games that needed the same treatment, I've decided it's
time to do just that.

So, this commit throws out all of Net's previous redraw code, and
replaces it with something in the more modern style I usually adopt in
new puzzles. The new draw_tile() doesn't read data out of the game
state willy-nilly; instead it takes a single word of bit-flags
describing everything about the tile it's drawing, and makes all its
decisions based on that word. And the main redraw loop constructs a
whole array of flags words describing what it would _like_ the grid to
look like, and then calls draw_tile() for every square whose word
doesn't match the one that was previously drawn there. (With the one
exception that the presence of the TILE_ROTATING flag in either the
old or new word forces a redraw _even_ if the two words are identical,
because that's how the move animation works.)

The new graphics look more or less the same at the default resolution
(there may be a pixel difference here or there but I don't think it's
noticeable if so), but now they scale up properly if you enlarge or
maximise the puzzle window.