Use FT_Outline_EmboldenXY if available
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
Don't forget to apply \be to the first/last row/column (again)

The implementation of \be was changed to xy-VSFilter's, which (like
VSFilter's) reads but does not write out the first/last row/column.
Add enough padding for \be to avoid clipping
Use just the necessary amount of padding for any \be value
Calculate \be using [0..64] value range (like VSFilter)

With this, our \be finally closely matches VSFilter's.

The only visible difference is clipping: we add proper padding
and output the whole blurred image, while VSFilter does not add
any padding and hence clips the blurred image too early.
Apply \be after \blur (like VSFilter)
Do the last \be pass using full [0..255] value range
[WIP] Fractional and scaled \be through Dirac deltas and a final box blur