Expose the RHI implementation in QRhi

If the application has access to the private header files (hence to the implementations such as `QRhiD3D11`) there currently seems no way for the application to grab the actual implementation from a `QRhi` instance to make use of the implementations directly.

I propose having a public `QRhiImplementation* implementation() { return d; };` method within `QRhi`. A similar thing is done in `QGuiApplication` currently through `QNativeInterface *QGuiApplication::nativeInterface()`.