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.