gsk: Add bounds member to RenderNode

gsk: Add bounds member to RenderNode

Instead of constantly recalculating this (especially recursively for
parents!) we do it only on construction, because everything is
immutable anyway. Also, most nodes had a bounds already and can
use the new parent member instead.

We also do direct access to the node bounds rather than calling
gsk_render_node_get_bounds in various places, which means
we do less copying.