Demos/skybox.scn : not working

[hugtalbot]

While working on cleaning scene warnings in #5183, I noticed that the skybox.scn example is not working anymore

[hugtalbot]

Error thrown:

[ERROR]   [GLSLShader] ERROR: Compilation of Fragment shader failed:
[ERROR]   [GLSLShader] From File: /data/Softwares/sofa/src/master/share/shaders/skybox.frag
[ERROR]   [GLSLShader] 0:4(1): error: `in' qualifier in declaration of `TexCoords' only valid for function parameters in GLSL 1.20

[ERROR]   [GLSLShader] ERROR: Compilation of Vertex shader failed:
[ERROR]   [GLSLShader] From File: /data/Softwares/sofa/src/master/share/shaders/skybox.vert
[ERROR]   [GLSLShader] 0:4(1): error: `out' qualifier in declaration of `TexCoords' only valid for function parameters in GLSL 1.20

[ERROR]   [GLSLShader] SHADER compilation failed.
[ERROR]   [OglFloat3Attribute(restPosition)] Variable "restPosition" NOT FOUND in shader "[ 'shaders/skybox.vert' ]"
[ERROR]   [OglFloat3Attribute(restNormal)] Variable "restNormal" NOT FOUND in shader "[ 'shaders/skybox.vert' ]"