Path tracing improvements

[xelatihy]

This issue tracks a list of possible improvements for the path tracer implementation. There should be no visible changes outside of the renderer internals.

• simpler eyelight make everything diffuse
• camera light with diffuse + specular
• use a point formulation
• consider using either recursion or an explicit loop but over a separate function, this allows us to "schedule rays"
• resolve coverage and envmap visibility at the camera not in the shaders --- the makes opacity less nice, but oh well
• add a background to the render options so that we always integrate explicitly, even in the case of no hits
• metallic -> reflective
• only shclick fresnel?

[xelatihy]

Listed in #1375