Add 'dot' line mode

[ybhvf]

This commit adds a new 'dot' line mode which produces small dots via very short lines. This is useful for plotting dithered art.

[abey79]

Thanks for your contribution!

This is an interesting mode, however I don't understand the rationale behind using the PIXEL_OFFSET multiplier. This offset of 5 is related to the big mode, where each pixel is made of a 5x5 pen widths square. If the intent is indeed to spread out the points, it probably should be a command argument.

Also:

• Tiny circles of diameter ~1-2 tenth of pen widths yield better result than small lines with most pen. I think it would be better for this mode (and I should likely do it every time a single pixel is drawn in the other modes btw).
• Do you have any examples to contribute for the doc?

[ybhvf]

Hah, there's no reason behind my using the PIXEL_OFFSET multiplier... It's leftover from big_mode(), which I copied and modified for dot_mode().

I've removed PIXEL_OFFSET from dot_mode(), and have updated it to create circle paths instead. I'll also add an example later today :)

[abey79]

This should read as follows, for correctness and clarity:

paths = [vp.circle(j * pen_width, i * pen_width, pen_width * 0.05)
                  for j, i in zip(indice_j, indice_i)]

In particular, the * pen_width is needed such that the grid is indeed scaled by the pen width.

[abey79]

FYI, I made some changes that will probably need you to rebase and force-push.