libass: updated to 0.15.1

Release 0.15.1

This is purely a bug fix and polish release, with no significant API or ABI changes.

The only API change is that ass_add_font is now declared to accept const char *. Previously it took only char *, but const has worked in practice since the very first standalone libass release.

Fix currently known 0.15.0 regressions:
Fix crashing on some ASS files with embedded fonts
Fix total font fallback failure with some fonts on Core Text
Fix embedded and memory fonts not actually being used unless ass_set_fonts was called after they have been added
Fix events assuming the largest xbord-value of a preceding event of the same frame during collision handling
Fix \fscx0 and \fscy0 making more text disappear than they should
Fix shift in vertical positioning of some right-to-left events with \an4-6, \pos, \move or Scroll effects
Fix imprecise output for \be with huge argument
Fix the --enable-large-tiles configure switch doing nothing
Fix output of ASM-enabled builds deviating from non-ASM builds
Fix ASM-builds on Haiku and 32-bit *BSDs
Overhaul x86 assembly for better performance and to allow ASM-enabled PIC-builds on 32-bit
Reduce temporary allocation in rendering
DirectWrite: request fonts on demand for better performance and improved compatibility with external on-demand font activation
Log libass version numbers upon renderer startup
Accept pointers to const as arguments to ass_add_font
Improve VSFilter compatibility:
Disregard zero area events in collision handling
Do not apply \fsp to vector drawings
Fix bug preventing cutting out glyphs from border & shadow with abnormal \fad values (or causing aliasing artifacts in some cases with normal values)


(adam)