Support arbitrary cursor width

[sphamba]

To deal with the cursor extending to wide characters (#7) and vertical-bar cursor (#33), change the rasterization technique to support arbitrary shaped cursor and smear.

Ideas

• describe the cursor/smear as a convex quad
• cursor corners closest to the next target are accelerated faster, corners farthest lag behind
• for each cell, check its position relative to the quad (outside, crossing edges, inside), and draw the cell accordingly
• different drawing rules depending on the number of edges passing through the cell