figure out what to do with intersecting bitmaps

[dankamongmen]

Right now, bitmaps are drawn in any order -- we're really designed (at the moment) for one bitmap on-screen at a time. We ought draw them in the z-order, from bottom to top. That might be sufficient for the bitmaps themselves. Unfortunately, that doesn't handle the following case:

• bitmap underneath
• text which is underneath
• a partially transparent bitmap

this is because the text is being drawn in the second text pass, which is the final rasterization pass. theoretically this calls for arbitrarily many raster steps. i obviously don't like that idea.

[dankamongmen]

While we're at it, we need to honor piles when we're rendering sprixels. Right now the sprixel cache is per-notcurses context, not per-pile. That's #1462.

[dankamongmen]

I think we can get pretty far by simply making sure we draw bottom-to-top, rather than the indeterminate order currently honored. that's #1575.