This is another example for a 2-texture shader.
So far, only separable blend modes are implemented.
The implementation is not optimized, with an
if-else cascade in the shader.
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.
This fixes the proper dependencies getting set up for generating
the shaders and only the necessary things getting rebuilt on
resources changing in gsk.
Spooky action at a distance is not really allowed in Meson, so the rules
to generate the SPV files should go in their own directory.
Tested by: Rico Tzschichholz <ricotz@ubuntu.com>
