Preserve3D baking space

[kvark]

We bake each preserve-3D stacking context in screen space as of a43b0c2 in order to get proper text rendering and anti-aliasing of the contents. However, this only works when preserve-3D forms isolated branches of the SC tree. This appear to not always be the case...

Consider the following tree: a 3D context A may contain 2D context B that contains another 3D context C. Baking of C should not be made in screen space but rather in B space, since B will participate in plane-splitting with A contents/children and is rotated in general case.

The quality-focused solution would probably involve having separate baking paths for screen space versus local space, and we'll fall-back to the latter if former is not an option (like in the example above). This is unfortunately going to increase the implementation complexity.

cc @glennw

[glennw]

We have some support for this now - each Picture can specify a RasterizationSpace internally. However, we need to expose this properly and make sure it works with clip masks etc.

[kvark]

Fixed by #3030