GeometryLight reduces effect of HDRILight

[mnaehrig]

Following image was rendered via pathtracing. It contains just a single HDRI light and simple geometries with Principled materials:

If we add a geometry light (small sphere with Luminous material), the overall brightness is significantly reduced:

We found out that this issue was introduced with commit cbf9d26. So it can't be reproduced with OSPRay 2.7.1 and earlier. A possible fix is to change computation of selectionPDF in VirtualLight.ispc from:

const uniform float selectionPDF = rcp((uniform float)pathContext.numLights - w->pathtracerData.numGeoLights);

to:

const uniform float selectionPDF = rcp((uniform float)pathContext.numLights);

But currently I'm not sure if this change has unexpected side effects.