[The Next Week] Image texture is gamma compressed twice

[stal12]

Telling whether a random image employs linear or sRGB color values is usually a hard task. However, linear space is certainly the rarest.

In the more common case that the texture image is gamma corrected already, color values are gamma compressed a second time in the write_color function, and the resulting texture appears brighter than the original image, like in the example from the book.

I suggest to gamma decode colors after the image is loaded, or at least to mention the issue because it is very common.

[trevordblack]

This issue is relevant to book 1, so I'm putting it in v4.0.0-book1 milestone