Various changes, mostly graphical

[robx]

This has a couple of straightforward and a couple of rather experimental changes.

• reword a lot of error messages, to get rid of "plural" mostly
• fix the shakashaka triangle drawing to align short triangle edges with given shaded cells
• use devicePixelRatio when available to
  1. draw grid lines with a width that's a multiple of actual pixel size
  2. draw grid lines with width below 1 at low resolutions on high dpi displays
• change the dashing count to a minimum of 4 (from 3 before)
• draw the chassis (frame around the puzzle) towards the outside, instead of centered; this should mean that cells next to the edge of the grid are more square; e.g. noticably masyu clues will appear less close to the edge

This PR is live for testing at https://garp.vllmrt.net/scratch/various.

Todo:

• loops (drawn with width lw) sometimes aliase to different widths vertically versus horizontally, particularly at low cell sizes

[robx]

I've split out two non-ready features (line width and chassis) into #166 and #167.