I see 'out of memory' errors and crashes inside libvulkan when
creating nodes that have empty bounds and end up in the fallback
paths, like a shadow around an empty text node. Prevent this
by not creating text nodes in that case.
Copy the PangoCairoRenderer into GTK+, rename it to GskPangoRenderer,
and strip it down far enough to build without private pango apis.
This means we currently don't support hexboxes or shapes.
Currently, this lives in gtk, but it might be nicer to put it
in gsk eventually.