Support colored emoji/symbol font as fallback

Follow-up of QTBUG-45811

Currently if a font doesn't support a certain character, the character is drawn using a fallback font that supports the specified char. This character will then be rendered only in monochrome.

If for example there is an emoji character inside a text rendered with Arial, it will be drawn in monochrome, even though the fallback font supports colors.

This can be seen in the example from QTBUG-71928.