Accelerated imperative painter

One thing lacking in Qt at the moment is a way to do high-quality imperative painting without going via software rendering, but instead building on top of the RHI. For instance, the canvas API in Qt Quick is implemented using QPainter, which means it involves expensive uploads of texture data for every frame, and for many use cases it does not perform well enough to be useful.

To fill this gap, we need to investigate what the different options are (paint engine based on RHI, importing external renderers, etc.) and ideally end up with something that can be used from both Qt Quick and for drawing directly to a surface like QRHIWidget, increasing performance but without forfeiting any quality or antialiasing etc.