AtlasEngine: Fix a OOB read when rendering PUA chars

[lhecker]

When no soft fonts are set up but we're asked to draw a U+EF20, etc.,
character we'll currently read out-of-bounds, because we don't check
whether the soft fonts array is non-empty. This PR fixes the issue by
first getting a slice of the data and then checking if it's ok to use.

This changeset additionally fixes a couple constinit vs. constexpr
cases. I changed them to constinit at some point because I thought
that it's more constexpr than constexpr by guaranteeing initialization
at compile time. But nope, constinit is actually weaker in a way,
because while it does guarantee that, it doesn't actually make the
data constant. In other words, constinit is not .rdata.

[DHowett]

this build is broke, fyi

[zadjii-msft]

(still doesn't build)

[lhecker]

I can't say I have seen a

LNK1169: one or more multiply defined symbols found

because I failed to include a header before. I'd understand if the definition was missing, but a duplicate symbol? Huh. Well, it should be fixed now!