Is it possible (plugin?) to use a camera input (solidview or materialview) in the compositor as a preview? While in the compositor, the usual input is the renderlayer, but when compositing, mostly when checking masks or rotoscoping, having a simple preview instead of a final render as an input is quite good (and fast) enough, so, instead of having to press f12 every time I move to other part of an animation to check, is there a way to use a camera output instead of render layer as a kind-of proxy to speed up. Yes, I know you can render the viewport into a sequence or videofile and use a moviefile input, but being a workaround for a specific situation, it's doesn't answer the general question.
If you're using 2.8x, you can change the render engine to 'Workbench.'

It's the equivalent of a viewport render in previous Blender versions, except it ports into Blender's rendering system, and therefore the compositor.  It should render a frame about as quickly as the viewport, and gives you to extra options (depth of field, outline, basic shadows, etc.)