Fix Canvas3D_conformance_context_attribute_preserve_drawing_buffer conformance test

The Canvas3D_conformance_context_attribute_preserve_drawing_buffer conformance test uses 2D canvases in a way that is not supported by Qt. This test should be fixed to check preserveDrawingBuffer handling some other way.

Low priority, as we do already have autotest for preserveDrawingBuffer in Canvas3D autotests.