Support path element combinations

Note: this is not handled correctly by Qt LottieAnimation nor by lottietoqml

If there are multiple shape elements in the same group, they should be combined into a single path, possibly creating holes, according to the Fill's fill rule. The path direction is significant, as set by the "d" (path direction) property. Behavior matches QPainter/QPainterPath and QuickShapes/ShapePath.

Example: omg.json, the "O" letter currently has no hole.

Also the "mm" (path merge) element is related, currently unsupported.