gsk: Remove gsk_render_node_get_size()

gsk: Remove gsk_render_node_get_size()

In the brave new world of refactored render nodes, this function doesn't
really make any sense anymore. We could turn it into a vfunc, but I
don't think it's useful.

Especially because even in the brave old world, this function was
causing a vastl overallocation of nodes when the GL renderer needed render
targets.