Progressive Rendering, Gaussian Splat primitive support

Added support for progressive rendering described as the following:
Frames: single pass of the fragment shader - progresses each ray by a single bounce
Images: combination of MAX_BOUNCES number of frames - is a single complete pass of the monte carlo path tracer
History: sum of all images that have occurred with the same scene parameters - the final render

Added support for Gaussian Splat primitives:
Uses a modified version of the collision algorithm described in Stochastic Ray Tracing of Transparent 3D Gaussians
Gaussian Splats are native primitives in the same way they are in my CPU path tracer