Use physical light units for lights

[dmnsgn]

Describe physically based lighting Attenuation

[vorg]

We should probably align units with gltf spec https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_lights_punctual/README.md

Brightness of light in. The units that this is defined in depend on the type of light. point and spot lights use luminous intensity in candela (lm/sr) while directional lights use illuminance in lux (lm/m2)

[dmnsgn]

Which slightly differs filament:

Light type	Unit
Directional light	Illuminance ($lx$ or $\frac{lm}{m^2}$)
Point light	Luminous power ($lm$)
Spot light	Luminous power ($lm$)
Area light	Luminous power ($lm$)
Image based light	Luminance ($\frac{cd}{m^2}$)

where point/spot use luminous power in lumen instead of luminous intensity in candela (lumens per square radian):

Photometric term	Notation	Unit
Luminous power	$\Phi$	Lumen ($lm$)
Luminous intensity	$I$	Candela ($cd$) or $\frac{lm}{sr}$
Illuminance	$E$	Lux ($lx$) or $\frac{lm}{m^2}$
Luminance	$L$	Nit ($nt$) or $\frac{cd}{m^2}$

[vorg]

Reminder to self: balance accuracy with developer experience (based on now fiddly DOF/FocalLength is).