render: We use CHECKED now, not ACTIVE

Update docs and custom rendering of checkmarks and radio buttons
stylecontext: fix icon-source extents computation

for the builtin image or invalid matrix case. We used to return invalid
memory in those cases. And that was not good.
css: Add "-gtk-icon-source: builtin"

... and make it the default. This takes over the meaning from "none" for
this property in that it draws the fallback builtin image.
"none" now literally means no image will be drawn.