More work on text nodes

More work on text nodes

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.