ctx,st3m: increment the frame-clock of the texture cache context

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.