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
Minor refactor as preparation for LayoutResX/Y

Reduces the diff for the next commit. No functional changes.
Implement LayoutResX/LayoutResY

This allows ASS scripts to be independent from video resolution.
Do font supersampling the VSFilter way

To avoid jerking when font size scale is animated caused by grid
fitting, we intentionally request an overly large font size from
FreeType and downscale the outline afterwards.

Currently this large font size is fixed at 256.
This commit changes it to 64 times the actual wanted font size,
which is what is used by VSFilter. The advantages are:
  * Compatibility with VSFilter.
  * Support for font hinting, even if at 64 times the size.
    Knowing this coefficient, scripts can compensate
    for it by correspondingly decreasing \fs values.

This improves upon commits 7af7801 and c207000.

The drawback is that jerking is reintroduced when an mplayer
window with libass EOSD is resized. Jerking could be avoided
by scaling \fscx and \fscy instead of \fs. Pro: this would make
the exact look of the font completely resolution-independent.
Con: this would play badly with font hinting. In any case,
this commit makes no attempt to do this.

ensure_font_size, unused since 7af7801, is removed in favor of
clipping the font size just before passing it to FreeType, as
discussed in issue #46. The mysterious height*2 check is dropped.
Scale \fscx/\fscy instead of \fs from storage to display resolution

This prevents jerking on media player window resize.