Some BLFont APIs belong to BLFontFace

[neslib]

There are some BLFont API's that don't apply to a specific font, but to the face used by the font.

For example, the BLFont::mapTextToGlyphs API is independent of the font, but only depends on the face that is used by the font.

It's fine to have this API at the BLFont level, but it would be nice to have a BLFontFace::mapTextToGlyphs API as well, so we can use it without having to create a BLFont instance.

[kobalicek]

This is right. I will definitely think of this in upcoming weeks as text is now a priority feature.