Unified handling of core shader generation defines

[mvaligursky]

In a move towards define driven uber-shaders:

• all core shader generation defines are handled in a unified way, mostly outside of the generators themselves, and are supplied as options. (GAMMA, TONEMAP, FOG, RENDERPASS, ..)
• removed gamma1_0 and gamma 2_2 chunks, and created a single gamma chunk driven by a define
• removed fogExp, forExp2, forLinear, fogNone chunks and create a single fog chunk driven by a define

Added a Test/Global Shader Properties example:

• hidden from published example, only for local testing (and so it's not very pretty)
• renders instances of all shaders generators (sky, gsplat, lit-shader, standard-material, particles, shader-material)
• enables tonemapping, fog and gamma to be changed to validate its functionality
• currently gsplat does not support fog, all else works