Changes to `TextFont`'s `font_size` and `font` fields

TextFont's font field has been changed from a Handle<Font> to a FontSource, and its font_size field has changed from an f32 to a FontSize.

FontSource has two variants: Handle, which identifies a font by asset handle, and Family, which selects a font by its family name.

FontSource implements From<Handle<Font>>, migration of existing code should only require calling into() on the handle.

Font texture atlases are no longer automatically cleared when the font asset they were generated from is removed. Even after the asset is removed, the font is still accessible using the family name with FontSource::family. Removing the text atlases naively could cause a panic as rendering expects them to be present.

For font_size, the migration is to wrap the f32 value in a FontSize::Px(...).

Concretely:

TextFont {
    font: asset_server.load("FiraMono-medium.ttf"),
    font_size: 35.,
    ..default()
}

becomes

TextFont {
    font: asset_server.load("FiraMono-medium.ttf").into(),
    font_size: FontSize::Px(35.),
    ..default()
}