Migrate to st-flexipatch with sixel support

Switch from manually patched st to st-flexipatch (bakkeby/st-flexipatch).
This uses preprocessor directives in patches.h to toggle patches at
compile time, making it much easier to add/remove features.

Enabled patches:
- alpha (transparency)
- anygeometry, anysize (dynamic borders)
- blinking_cursor
- boxdraw (box-drawing chars)
- clipboard
- columns (no text cut on resize)
- copyurl
- externalpipe
- font2 (fallback fonts)
- iso14755 (unicode input)
- ligatures (HarfBuzz)
- newterm (spawn in cwd)
- scrollback + scrollback_mouse
- sixel (NEW - for manga-tui image support)
- swapmouse
- sync
- xresources + xresources_reload

To toggle patches: edit patches.h and rebuild.
See README.md for documentation on the preprocessor system.