Merge pull request #2 from mrdoob/dev

Update Dev
A simplied version of the total Reayleigh scattering to works on browsers that use ANGLE
Fix Sky Shader. Should now work on Windows and Mac (#5602)

- use simplified Rayleigh function for performance and ANGLE
- add back original in case anyone wishes to experiment wavelengths
Merge remote-tracking branch 'zz85/fix_sky_shader' into dev