Various mathematical transformations

[therontarigo]

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Generally an improvement:

• [ ] cross(-a,b) or cross(a,-b) or -cross(a,b) -> cross(b,a) (related issue: sign cancellation)
• [ ] Left vs right multiplication: transpose(mat)*vec -> vec*mat vec*transpose(mat) -> mat*vec
• [ ] transpose(a)*transpose(b) -> transpose(b*a)
• [x] distance(a,b) -> length(a-b) (completed in 40dae3c)

Only if there's a flag to turn it off (otherwise could hurt compression):

• [ ] sqrt(dot(a,a)) -> length(a)
• [ ] min/max simplification (related issue: sign cancellation) min(-a,-b) -> -max(a,b) -max(a,-b) -> min(-a,b) and so-forth.
• [ ] Common cross-product expansions: cross(vec3(1,0,0),a) -> vec3(0,-a.z,a.y) cross(vec3(0,1,0),a) -> vec3(a.z,0,-a.x) cross(vec3(0,0,1),a) -> vec3(-a.y,a.x,0) ( perhaps generalized to cross(vec3(d,0,0),a) -> d*vec3(0,-a.z,a.y) etc. (when d is known to be scalar type), though this doesn't seem common )

Expected impact on precision: none, driver shader compilers typically perform similar transformations anyway.