Passing non-const data is inconvenient and error-prone, as data can't be
used again after it was modified by Nano SVG, which resulted in the
button using SVG bitmap not working any longer in the toolbar sample
after recreating the toolbar.
So make it easier to do the right thing, while still keeping the
overload taking non-const data for the situations when avoiding an extra
copy is worth it.
Add nanosvg submodule and use it in the generic implementation of this
function.
This is incomplete yet and, notably, doesn't cache the rasterized
images, but already shows that using SVG images works (at least in the
toolbar sample).
