Allow building as DLL on Windows

Patch by jon_y on IRC.
Improve font mismatch message

Example for an old message:

 [ass] fontconfig: Selected font is not the requested one: 'DejaVu Sans' != 'Wingdings'

it was hard to tell which was the selected and the requested font.
Also, it's not really clear what's the problem at all. Why would it
select a different font? Obviously, the issue is that it can't find
the font in the first place.

Now it prints:

 [ass] fontconfig: cannot find glyph U+006C in font 'Wingdings', falling back to 'DejaVu Sans'

Or if the code parameter for select_font() is 0:

 [ass] fontconfig: cannot find font 'Wingdings', falling back to 'DejaVu Sans'

I'm not sure if this message is really accurate in all cases. It's
possible that there are more reasons for failure. But all things
considered, this should be easier to understand.
Fix blur scaling

Commit 0e1702ad7a6a827d "Add ass_set_storage_size and fix related scaling issues"
attempted to fix blur scaling. This breaks blur scaling for players
which don't call ass_set_storage_size().

This commit reverts the default behavior to the old behavior. The
behavior when ass_set_storage_size() is called should be unchanged.
Fix \r style lookup

Make \rSTYLENAME with an invalid STYLENAME fall back to
line style rather than to Default. This fixes issue #104.
Parse style names like VSFilter does

Trim '*' from the start of style name fields of Style
and Dialogue lines and normalize the case of "Default"
in the style name field of Dialogue lines.
Allow different ASS_Library for ASS_Track and ASS_Renderer

Remove the explicit check whether the ASS_Track and ASS_Renderer were
created from the same ASS_Library object. Logging will not be entirely
consistent (some log messages go to the ASS_Track library when
rendering), but otherwise it works.

ASS_Library also contains embedded fonts added with ass_add_font().
It looks like the renderer will use the fonts from the ASS_Renderer
library object.
Remove unused variables
Don't mutate input buffer for ass_read_memory()

Fixes google code issue #88.

process_text() garbles the memory as the text is parsed.

This also fixes that the user provided buffer implicitly had to
be null terminated. This wasn't obvious, because the caller
passes in the buffer length. libass ignored the buffer length
(unless a codepage was provided and iconv was enabled), and
happily read past the end of the buffer.

It would be much nicer if the parsing code would be fixed,
instead of just copying the input buffer. Maybe one day.
Fix OS/2 usWinDescent/usWinAscent for quirky fonts

Some fonts stuff a signed, negative value into this unsigned
field. This usually causes very small and wrongly positioned
rendering. Also handle usWinAscent similarly, just in case.

Fixes issue #106.