Calculate bounding box by traversing Paint tree if there is no expicit box or base glyph

[justvanrossum]

Need to check the spec for details. blackrenderer currently relies on having a ClipBox or a base glyph bounding box.

[justvanrossum]

"no base glyph" should probably be read as "empty base glyph".

[anthrotype]

the bounding box for an "empty base glyph" would be (0,0,0,0) but I think you actually want it unbounded, i.e. (float("-inf"), float("-inf"), float("+inf"), float("+inf"))

[justvanrossum]

That would be a problem for the high level render code that uses bounding boxes to calculate a decent default canvas size.

I think it should fall back to traversing the Paint graph to obtain the actual box in that case, as per the title of this issue.