`FontAtlas` changes

The texture atlas layout for font atlases is no longer stored as a separate asset. Instead, it is stored directly in the texture_atlas field of FontAtlas.

The TextureAtlasLayout parameters of FontAtlas's new and add_glyph_to_atlas methods have been removed.

FontAtlas::add_glyph's offset parameter has been changed from an IVec2 to a Vec2

GlyphAtlasInfo's texture_atlas and location fields have been removed, replaced by rect and offset fields.

The size field has been removed from PositionedGlyph. The glyph’s size can now be obtained from the Rect stored in the atlas_info: GlyphAtlasInfo field.

GlyphAtlasLocation::offset is now a Vec2.