This commit adds visualization to the selection in curves sculpt mode.
Previously it was only possible to see the selection when it was
connected to a material.
In order to obstruct the users vision as little as possible, the
selected areas of the curve are left as is, but a dark overlay
is drawn over unselected areas.
To make it work, the overlay requests the selection attribute and then
ensures that the evaluation is complete for curves. Then it retrieves
the evaluated selection GPU texture and passes that to the shader.
This reuses the existing generic attribute extraction system because
there currently wouldn't be any benefits to dealing with selection
separately, and because it avoids duplication of the logic that
extracts attributes from curves and evaluates them if necessary.
Differential Revision: https://developer.blender.org/D15219
