GskRenderNode: Use C99 flexible arrays to avoid extra allocation

GskRenderNode: Use C99 flexible arrays to avoid extra allocation

Instead of a separate allocation for any arrays in the render node
we allocate these as part of the render node itself, using C99
flexible arrays.

This leads to less allocations, which is nice, but the major reason
for this is that it allows us to change the allocation scheme further
in the future. For instance, we want to do stack-like allocation so
that all the render-nodes for an entire frame are allocated in one
(or a few) chunks.