Custom material output definition is inconsistent

The definition of the fragment shader output is inconsistent. Shader generator adds out vec4 fragOutput to all generated shaders, but custom materials specify that they need to add out vec4 fragColor to the shader. Additionally custom material generator expects fragColor. All our custom materials add the fragColor. Therefore there are multiple output variables added to a fragment shader.

This doesn't work in GLES 3.2, which specifies that all all output variables need to be bound to fragment color.

The definition of the custom material output variable needs to be consistent. We should either specify what variable to use and the material not using it should not compile, or we specify that the user must specify the output variable and we do not add any output variable by default.