Remove bitmap restriding

It turns out we don't need this.
Fix change detection when text is clipped to screen

The glyphs are clipped before combining, so the combined bitmap can
always have the same position, size, and address. This breaks the
change detection. Or at least I think that's what happens.
Add BorderStyle=4, background color for text

This is somewhat similar to BorderStyle=3, but with a number of
differences. Mainly, this new BorderStyle doesn't create overlaps
within an event.

Closes #105.
Merge pull request #100 from astiob/stroke

Use FT_Outline_EmboldenXY
Close gaps in borders of self-intersecting glyphs

If the glyph is self-intersecting, so are the stroke
borders produced by stroking it. Normally, there is an
"inside" and an "outside" stroke border, but they swap
roles around self-intersections, the "inside" one going
on the outside while the "outside" one stays inside.
Currently, we take only the "outside" border, which thus
hides behind the main glyph around self-intersections.

Keep both stroke borders instead.

NOPE: this gives only a thick path around the edges
of the glyph, which breaks blur.
stroke_outline: merge original and stroked outline to fix blur
fix_freetype_stroker: ensure overall clockwise direction

The stroked outline's overall direction is always clockwise, so we
need this to be able to correctly add it to the original outline.
Disable fix_outline for debugging
stroke_outine: output outlines for debugging