v3.0.3—true to form

No big changes here. Sixel quantization has been rewritten; it is now substantially faster, but has taken a quality hit on some images. I'll be working on rectifying this for 3.0.4 (see #1452, #2503, and #2516).

• Eliminate garbage input in XTerm on startup when Sixel is not supported (#2517)
• Count sixel properly in bitmap stats (#2507)
• Fix [box] demo when there is no available pixel geometry (#2505)

there might have been a few other things in there, but they were small. Most work was related to improving SIxel quantization.