Restrict shared rendering to the main thread

KSvg tries to share a renderer between instance. QSVGRenderer is not thread safe, and we have usage crashing. In particular in an async impage provider.

Unfortunately it is not as simple as making one per thread, as sometimes KSvg renderers are created from updatePaintNode on the render thread, which is safe to do as the main thread is paused.

This patch uses the shared renderer based on the thread afinity of the KSvg object. Only the main thread is shared, as any other thread usage is probably short lived.