All the contexts share the texture and eid handling of the "master" ctx
context, which is within the rasterizer of the rendering context for
framebuffer 0. For the age-tracking to work right we need to have a logical
clock tied to the use of textures, this adds a small convenience function to
the ctx implementation to inject an appropriate increment of this clock per
rendered frame. This makes the ejection policy of discarding a least frequently
used eid, which works well with eids created on demand and cached by an
identity.
