I tried to cross compile harfbuzz for Windows and an error was generated because `DWrite_1.h` was not found.
This happened because the filesystem is case sensitive and for this reason that include file was not found.
The right name of the file to be used is `dwrite_1.h`, with all letters not capitalized:
https://docs.microsoft.com/en-us/windows/desktop/api/dwrite_1/
I also verified in the installation of VS2017 with Windows Kit v10 and in that place it was also lower case.
So, in my opinion it should be better to change this.

Also checks if DWriteCreateFactory() has been executed successfully.

It makes sense to disable this code these if editing is not needed.
However, this is also necessary to correctly display emoji sequences
in right-to-left direction.  For that reason, don't auto-disable it.

Part of https://github.com/harfbuzz/harfbuzz/issues/1652

Part of https://github.com/harfbuzz/harfbuzz/issues/1652

Don't know who would want when why.  But makes sense to have.

Part of https://github.com/harfbuzz/harfbuzz/issues/1652

Part of https://github.com/harfbuzz/harfbuzz/issues/1652

Part of https://github.com/harfbuzz/harfbuzz/issues/1652

https://github.com/harfbuzz/harfbuzz/pull/1796

moved has () to hb_bimap_t from hb_inc_bimap_t
moved identity () to hb_inc_bimap_t
removed forward()

Saves another 2.5kb.

Part of https://github.com/harfbuzz/harfbuzz/issues/1652

Part of https://github.com/harfbuzz/harfbuzz/issues/1652

Saves another 12kb:

$ python ./gen-ucd-table.py ucd.nounihan.grouped.zip > hb-ucd-table.hh && make -j5 CPPFLAGS='-Os -DHB_TINY' -C ~/hb/build/src/  && size ~/hb/build/src/.libs/libharfbuzz_la-hb-ucd.o
INFO: Loading UCDXML...
INFO: Preparing data tables...
INFO: Generating output...
INFO:   Compression=1:
INFO:       Dataset=gc       FullCost=18612
INFO:       Dataset=ccc      FullCost=3550
INFO:       Dataset=bmg      FullCost=1548
INFO:       Dataset=sc       FullCost=17765
INFO:       Dataset=dm       FullCost=13325
INFO:   Compression=3:
INFO:       Dataset=gc       FullCost=10726
INFO:       Dataset=ccc      FullCost=2389
INFO:       Dataset=bmg      FullCost=1052
INFO:       Dataset=sc       FullCost=13669
INFO:       Dataset=dm       FullCost=7780
INFO:   Compression=5:
INFO:       Dataset=gc       FullCost=8274
INFO:       Dataset=ccc      FullCost=2055
INFO:       Dataset=bmg      FullCost=908
INFO:       Dataset=sc       FullCost=4073
INFO:       Dataset=dm       FullCost=7780
INFO: Done.

Part of https://github.com/harfbuzz/harfbuzz/issues/1652