Tracking pending ABI/API changes which aren't worth a break on their own and a "wishlist" in case we ever need to make larger API overhaul for some reason.

ABI break to be rolled up into the next one:

• v4++ fields, see PR #651

Small API cleanups:

• Remove treat_fontname_as_pattern from ASS_Style. It's a noop, but currently still used by MPlayer and in the past by mpv. Ideally MPlayer should receive a patch first. Unlikely to ever have been used by others.

API wishlist in case there needs to be larger break anyway:

• Split ass_set_margins into two distinct interfaces to allow pan&scan and use of any remaining monitor margins at the same time
• Prefix all public macros and enums with ASS_. Currently the image type in ass.h and almost everything from ass_types.h lacks such a prefix.
• Use more correct types, like size__t instead of int for element counts, correct signedness, don't use int, which is only >= 16bits for ass_library_version which already currently requires at least 32 bits, etc
• Return error codes or similar for all API which can possibly fail (instead of mostly just void now)
• Remove sizeof(ASS_Style) and sizeof(ASS_Event) from ABI and API, to make manual style and event creation and access more robust and less prone to cause ABI-breakages.
  E.g. we could mandate use of accessor functions instead of direct array access to be able to add new fields at the bottom. Or if we also want to be able to freely reorder fields and normalise set values we could provide setter and getter functions similar to fontconfig's FcPatterGet*.
  • furthermore, on platforms which do not record the minor library version of linked to dependencies, any kind of struct extensions can complicate things. Since neither the struct nor its members exist as symbols in the shared library the incompatibility with older minor versions is not detectable from the binary itself and requires keeping additional metadata around. Attempts to use incompatible versions at runtime will lead to out-of-bounds reads and writes.
• change how ass_set_font_scale is applied by default. Currently its default is to affect all events and there’s ASS_OVERRIDE_BIT_SELECTIVE_FONT_SCALE to apply it only to events looking like regular dialogue. This is unlikely to be desired and inconsistent with what we do for style overrides. Thus either fully match style overrides and only apply selectively/fully when enabled with ASS_OVERRIDE_BIT_SELECTIVE_FONT_SCALE(_FULL), or apply selectively by default and provide ASS_OVERRIDE_BIT_SELECTIVE_FONT_SCALE_FULL to apply unconditionally. to apply either selectively by default, or fully match ass_set_style_overrides
• make calling set_storage_size mandatory requiring an explicit opt-out via 0, 0 and if not called fail like for unset frame size to avoid accidental ASS incompatibilities
• support RGB output to (ease) support(ing) colour glyphs (see issue #381) (But how should the glyph's colour interact with colours specified by ASS? They are not supported by VSFilter)
• reconsider memory fonts being in ASS_Library (all other font stuff in in the renderer and attached fonts should be in the track)
• drop all deprecated API