[glTF] Support emissive texture

[footballhead]

emissiveTexture is loaded and passed to the StandardMaterial but is never referenced.

The purpose is outlined in 3.9.3:

The emissive texture and factor control the color and intensity of the light being emitted by the material. The texture MUST contain 8-bit values encoded with the sRGB opto-electronic transfer function so RGB values MUST be decoded to real linear values before they are used for any computations. To achieve correct filtering, the transfer function SHOULD be decoded before performing linear interpolation.

This would allow us to render the following correctly for example:

• DamagedHelmet
• BoomBox
• PotOfCoals