Half-pixel advance

[bgilbert]

When rendering bitmaps, we have the ability to use half-pixel advances:

• Graphics mode 1 uses double horizontal density, on the condition that adjacent pixels are not set. However, we could do multiple print passes with a half-pixel delta, e.g. when doing multiple strikes in HDR image printing #79.
• ESC J can advance the paper by half a pixel.

We could use this for:

• Banner mode #10
• bitmap mode, perhaps with a dense parameter
• image mode, perhaps with a dense parameter (xref Support automatically scaling images #81)

If we used true double density, we'd probably produce an image that's way too dark (except for #10). We could potentially shift alternate full rows by a half step to reduce grid artifacts, or try implementing double-density-aware dithering.