add label rendering

[mk30]

No description provided.

[mk30]

minor improvements to label rendering on branch https://github.com/peermaps/mixmap-georender/tree/label-improvements.

now looks like this with a large font size:

and like this with a small font size:

it's still hard to read at the smaller size. additionally, pieces of other labels are seen around some of the edges of a label's bounding box. it also seems like sometimes the text for a label is higher or lower inside its bounding box depending on whether there are any characters with low-hangiing bits. either way, there are many tweaks remaining.