Handle shader-decoded texture alpha correctly

[slimbuck]

This PR fixes a shader compile error resulting from user choosing the alpha channel of a decoded texture. (This is because the decoding step takes a vec4 and returns a vec3 so alpha doesn't exist).

Since there is never actual decoding of texture alpha (i.e. gamma textures don't correct alpha and RGBM/P/E textures use alpha channel to reconstruct RGB) we just add a passthrough for this case instead.

Also move textureSample.js chunk to the correct location.

[mvaligursky]

I just wonder - how come textures stored in sRGB don't gamma correct the alpha channel? If I'm reading this right.

[slimbuck]

if I'm reading this right.

Yep you are: alpha is just never stored in gamma space (presumably because it's not a color value).

[mvaligursky]

Ah, looking online, it certainly seems to be the case!