Skip useless memset() when copying a bitmap
shaper: add FriBidi error handling

It is unlikely, but FriBidi might not process a string correctly,
and we should handle that. Tested by making it fail always. This
should also fix some compiler warnings.
Merge pull request #102 from grigorig/fribidi-errorhandle

shaper: add FriBidi error handling
unistd.h is not needed
Move ASS_Shaper declaration to ass_shaper.h
Explicitly include ass_utils.h everywhere (and not config.h)
Fully fix compilation with MSVC/ICL

As before, this does not add any build system support:
a config.h file and a project must still be manually created
(or the compiler can be run manually instead of using a project).