- drw: minor improvement to the nomatches cache
- overhaul utf8decoding and render invalid utf8 sequences as U+FFFD.

Thanks NRK for these improvements!

- better scaling for occupied tag squares.
- draw statusline first to omitt some complicated calculations.

important:
- drw_rect: didn't use w and h, change the dwm code accordingly.
- drw_text: text is NULL is not allowed, use drw_rect().