demo changed to plot fps at 10Hz using braille

[Nick Black]

i think it really punches up the FPS plot! you can also see use
of the new NCPLOT_OPTION_PRINTSAMPLE if you look in the lower
right, showing the exact value of the most recent sample.
optimizations to plot drawing mean that despite plotting 10x
more frequently, we use less total cpu [flex].

also, a guest happened to see the "eagle" demo, and asked "are
those flying pickles"? turns out i somehow used the same palette
for "luigi" as i did "eagle", last december when i wrote them,
and the eagles have been the wrong colors this entire time.
that's fixed up.

finally, the "keller" demo added around 2.0.4 completes the set
of 25 demos:

```
specify demos via their first letter. repetitions are allowed.
default spec: ixezcydthnmbkgarwuvlsfjqo

ⓐ llglyph ⓑ ox ⓒ hunli ⓓ ragon ⓔ agle
ⓕ allin' ⓖ rid ⓗ ighcon ⓘ ntro ⓙ ungle
ⓚ eller ⓛ uigi ⓜ ojibake ⓝ ormal ⓞ utro
ⓠ rcode ⓡ eel ⓢ liders ⓣ rans ⓤ niblock
ⓥ iew ⓦ hiteout ⓧ ray ⓨ ield ⓩ oo
```

there was originally going to be a "pango" demo, but i decided
against dragging pango into the dependency stack (also, font
renders tend to look like shit in our low resolution, especially
before sexblitter came along).

26 isn't divisible by anything worth a damn, so we're going to
stay at 25 for the nice 5x5 table, lol.

anyway, if you haven't run notcurses-demo in a minute, please
give it a good run in your terminal of choice, and let me know
any problems you see before 2.1.0.

hack on! --nick

--
nick black -=- https://www.nick-black.com
to make an apple pie from scratch,
you need first invent a universe.