The code now follows gsk_rounded_rect_shrink() and with it the behavior
of the Cairo renderer and Webkit.
The old code did what the GL renderer and Cairo do, but I consider that
wrong.
I did not test Chrome.
Test attached
Move the resources of each renderer to its subdirectory.
We've previously done that for the ngl renderer, but it
is better to be consistent and do it for all the renderers.