Allow for custom layer blending/drawing

One interesting capability is to be able to write one's own layer blending, specially the way it combinates layers.

I myself would like to have the following setup:
-One layer has tiles in say, 2,3 or 4 shades of monochrome
-Another layer is used to select a palette for the tile above, and will replace each shade of monochrome with a colour, or alternatively, there is one layer for selecting each of the colours of the palette

and similar. This would require the capability to write one's own blending/layer drawing functions. It would be very useful though, specially for retro dev.

The issue names no files, tests, or entry points. Start by clarifying the scope and API for custom layer blending and drawing, including the monochrome-layer and palette-selection use cases; done would mean those combinations can be implemented and used for retro development.

Written by the indexing model from the issue text.