Initialize \pbo for each event

Regression in commit 1f5eb5eb.
Fixes issue #74.
Update version info and changelog
Update README
Fixed headers in profile and test
Fix premature parser exit on }{ in drawing mode

Fixes #75.
Implement fast quad-tree rasterizer in C and x86/SSE2/AVX2

Signed-off-by: Rodger Combs <rodger.combs@gmail.com>
Fix corner case: \move with identical start and end times
mystrtoll: don't truncate to int
Make \be animatable

VSFilter has supported this since version 2.39.

Use the raw floating-point value of the \be argument in the animation
formula, like xy-VSFilter has done since version 3.0.0.45 (404301a3).
Reset clipping mode on every rectangle \clip

Prior to this fix, both of the following:
    \iclip(0,0,9999,9999)\clip(0,0,9999,9999)
    \iclip(0,0,0,0)\clip(0,0,9999,9999)
hid the whole picture in libass. The correct behavior
in both cases is to display the whole picture.
Fix \1a, \2a, \3a, \4a with invalid argument

Reset to the initial color's alpha component, not red.
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.