With #200, we have a working sixel (and pixel generally) interface for both direct mode and rendered mode, but we're emitting garbage sixel. We want to be on at least the level of libsixel.

Some immediate problems:

• gaps between bands
• we're not supplying the image geometry (for warmech.bmp, libsixel emits "1;1;50;50 immediately following Pq
• we're not building up a color map ahead of time
• we need quantize down to a palette of 256 colors (xterm only supports 16 by default)
• we need fix up multi-scanline bands
• we're not using RLE
• we print many more bands than we ought
• we need to be setting the width field correctly (we currently always set it to 1)

NCBLIT_PIXEL must currently be considered non-working; we need several of these to get it to "experimental".