Add JOIN_R and JOIN_D flags, and extend effects of JOIN_*

Add JOIN_R and JOIN_D flags, and extend effects of JOIN_*

The new flags mark glyphs that join to the right and downwards.  The
effect of these, and a new effect of JOIN_U and JOIN_L, is to arrange
that getpix() treats pixels beyond the edge of the character cell as
repeating the last row of real pixels.

In practice, this means that a diagonal stroke touching the right or
bottom edge of the cell will now be drawn as though it turns into an
orthogonal stroke across the edge rather than as though it stops.  This
doesn't affect the outline of any existing glyph.

There is a more subtle consequence, which is that joining glyphs no
longer get edge hints on their joining edges.  I think that's an
improvement: such hints might move those edges so that they don't touch
the adjacent character, which would be rather unhelpful.