This commit introduces the Unicode placeholder image placement method.
In particular:
- Virtual placements can be created by passing `U=1` in a put command.
- Images with virtual placements can be displayed using the placeholder
character `U+10EEEE` with diacritics indicating rows and columns.
- The image ID is indicated by the foreground color of the placeholder.
Additionally, the most significant byte of the ID can be specified via
the third diacritic.
- Underline color can be optionally used to specify the placement ID.
- A bug was fixed, which caused incomplete image removal when it was
overwritten by another image with the same ID.
Also roundtrip all characters in the Cf category.
Characters with the DI (Default Ignorable) property are now
preserved but not rendered and treated as zero-width
as per the unicode standard.
See https://www.unicode.org/faq/unsup_char.html
Saves a couple of KB of RAM and is more flexible in terms
of max number of allowed chars, although for large numbers one really
needs a hash for fast lookups.
Rendering of sequences containing zwj is still not implemented, since it
can cause the collapse of an unbounded number of characters into a
single cell. However, kitty at least preserves the zwj by storing it as
a combining character.
Use a level of indirection to store combining characters. This allows
combining characters to be stored using only two bytes, even if they are
after USHORT_MAX
