Update to Splat engine example + small fixes

[mvaligursky]

• clone centers for each instance, as its ownership gets transferred to a worker
• generate unique shader id based on vs and fs
• updated visuals, to test integration with 3D scene and work of multiple splats together

Splat example:

Splat-many example:

[LeXXik]

Looks great! The lighting on both Wills breaks the immersion, though :)