* feat(canvas): implement half block marker
A useful technique for the terminal is to use half blocks to draw a grid
of "pixels" on the screen. Because we can set two colors per cell, and
because terminal cells are about twice as tall as they are wide, we can
draw a grid of half blocks that looks like a grid of square pixels.
This commit adds a new `HalfBlock` marker that can be used in the Canvas
widget and the associated HalfBlockGrid.
Also updated demo2 to use the new marker as it looks much nicer.
Adds docs for many of the methods and structs on canvas.
Changes the grid resolution method to return the pixel count
rather than the index of the last pixel.
This is an internal detail with no user impact.
