This commit takes several steps towards rendering text
like we want to.
The creation of the cairo surface and texture is moved
to the backend (in GskVulkanRenderer). We add a mask
shader that is used in the next text pipeline to use
the texture as a mask, like cairo_mask_surface does.
There is a separate color text pipeline that uses the
already existing blend shaders to use the texture as
a source, like cairo_paint does.
The text node api is simplified to have just a single
offset, which determines the left end of the text baseline,
like all our other text drawing APIs.
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.
