Support rendering to QImage via QQuickRenderControl without having to use private APIs

Currently to render offscreen using QQuickRenderControl, you need to QQuickWindow::setRenderTarget - and all available {{QQuickRenderTarget}}s are native, so you need 4 different native implementations.

Qt should expose a cross-platform QQuickRenderTarget so that I can render to this then read back the results as a QImage. This is for e.g. rendering QML offscreen as image frames into a video.

The test tst_RenderControl::renderAndReadBackWithRhi() in https://github.com/qt/qtdeclarative/blob/v6.0.0-beta5/tests/auto/quick/qquickrendercontrol/tst_qquickrendercontrol.cpp#L137 does exactly what I want. And the comment in that test is exactly what this bug is about - why do application developers have to do 4 separate native implementations to make this work?

// What comes now is technically cheating - as long as QRhi is not a public
// API this is not something applications can follow doing. However, it  
// allows us to test out the pipeline without having to write 4 different
// native (Vulkan, Metal, D3D11, OpenGL) implementations of all what's below.