Manual-related changes

[bratpeki]

In parallel with zynaddsubfx/user-manual#22.

[bratpeki]

Currently generating images:

* add-synth.png
* amp-control.png
* bank-read.png
env.png
filter-analog.png
filter-formant.png
filter-formant-vis.png
filter-svf.png
* info-tray.png
* kit.png
lfo.png
logo.png
* osc-overall.png
osc-tight.png
* pad-synth.png
part-settings-button.png
part-settings-controllers.png
part-settings-instrument.png
part-settings-labelenable-close.png
part-settings-labelenable.png
part-settings-portamento.png
part-settings-scale.png
part-settings-selection.png
settings-bank-roots.png
settings-global.png
settings-preset-roots.png
* sub-synth.png

Images with * are used in the manual.

[bratpeki]

The manual requires, but currently doesn't script-generate these images:

* virtual-keyboard.png (implemented as footer.png)

[bratpeki]

The script now additionally generates macro-learn.png and mixer.png.

[fundamental]

Try changing deps/nanovg/src/nanovg.c:37's constant of NVG_INIT_FONTIMAGE_SIZE from 512 to 1024 (I'm 99% sure this fixes the issue and was an ✨ Adventure ✨ to find).