FlxSpriteGroup fixes

Fixing transforms for angle and alpha in FlxSpriteGroup to keep properties of children relative to the parent group.

This still needs additional work to make changes in group angle adjust the ``x`` and ``y`` coordinates of children by rotating them around the group's origin, but this improves the strange behaviour of setting the group's angle adding that new angle to the children's angles rather than adjusting them by the offset.

Group alpha changes now respect the relative alphas of the children, provided the children alphas are not changed directly. To make this work as expected, some more work would need to be done to calculate the actual children alphas at draw time, rather than changing them at the point that the parent's alpha is changed.