Density rendering

[skywolf829]

Add support for splatting gaussian density to screen instead of RGB.

Requirements

• Add a boolean flag in the forward pass of the python/CUDA code for the render for a density variable which defaults to False (so as to not disrupt existing code).
• Instead of normal compositing, the density should accumulate for all gaussians splatting on each pixel. In other words, the pixel should never reach some final accumulated value and finish processing further gaussians. This may make the render with density actually slower than normal.
• The gaussian value (at a pixel) multiplied by the alpha should be the corresponding "density" for each gaussian, and this should be added for each gaussian splatted to a pixel.
• Result may need to be normalized [0,1.0] or [0, 255] for final display to screen in the app.

[skywolf829]

Closing to move to the backend repo.