Modify expression set in dinsey editor inputs

The color which should be found in the input of the disney material
editor must be transformed from sRGB to linear RGB.

Also the texture expression should be different. It should look like
texture("path", ...).

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
You can use `qcolor_to_color3()` defined in `src/appleseed.studio/utility/interop.h`. Also, you can use the `Color3f` typedef:

    const Color3f = qcolor_to_color3<float>(color);