Don't crash on \fscx0

Freetype can return a bounding box with all fields set to INT_MIN if an
outline with all points set to 0 is used. This can happen e.g. with
\fscx0, but also in more complicated cases. (In the original crashing
sample, this was probably caused in combination with an embedded font.)

Such a bounding box causes libass to crash, because it will enlarge the
combined bitmap bounding box to a ridiculous size.

Just skip outlines that have en empty bounding box. This is probably
the correct thing to do, and won't pass INT_MAX down to other parts
of libass.
Actually export ass_library_version symbol

Patch by chadr...@gmail.com.

Fixes libass#58, GC#117.
Fix change detection in presence of vector clips

Fixes this test case:

Dialogue: 0,0:00:00.00,0:00:02.00,,,0,0,0,,{\fs50\pos(250,250)\iclip(m 0 0 l 400 0 l 400 220 l0 220)}Hello
Dialogue: 0,0:00:02.00,0:10:00.00,,,0,0,0,,{\fs50\pos(250,250)\iclip(m 0 0 l 400 0 l 400 230 l0 230)}Hello

The problem here is that the rendered output bitmap list itself does
not change, only its contents (due to the different vector clip).
ass_render_frame() will not set *detect_change correctly, and an
application using this flag (like mplayer or mpv) will not update
the screen as needed.

Fix this with a very cheap hack: always report a full change if
there's a vector clip. This is basically an emergency fix until
we have a proper way to detect the change.
Merge pull request #61 from wm4/fix2

Fix change detection in presence of vector clips
Fix API documentation of change detection
Better fix for zero area outlines

Signed-off-by: wm4 <wm4@nowhere>

Commit 04b51c2d broke BorderStyle=3 with text that had spaces.
Very rough, completely non-functional initial attempt at IIR blur
Fixed one bug
Screwing around
Callee-saved shit on Win64