Currently all Prism RenderTargets are tied to graphics devices. One problem with them is that they can become invalid at any time, e.g. after screen lock, restoring from a sleep state etc. This request is for a RenderTarget backed by a RAM buffer, something like Swing's BufferedImage.

Such RenderTarget is needed to support HTML5 Canvas properly. Whatever is drawn on a canvas, we should be able to repaint at any time in response to a scroll or repaint event, so we need an offscreen buffer behind the canvas. Currently we use RTTexture, and when it is invalidated, everything is lost. Hence this request for a permanent buffer.