Qt3D: Add optional QRect parameter to QRenderCapture::requestCapture()

It would be helpful if QRenderCapture::requestCapture() could accept a QRect as an optional parameter in order to capture only part of the buffer. It would facilitate functionality that will also work on machines where the new QBufferCapture can not be applied because of an older version of OpenGL.

In other words, it would be nice to have the following function in QRenderCapture API: Qt3DRender::QRenderCaptureReply *requestCapture(int captureId, const QRect& rect); where rect is the rectangle to capture.