Support OSL code generated from image shaders in MaterialX 1.38.10

[AdrienHerubel]

This PR adds support for the OSL code generated for image shaders by MaterialX 1.38.10.

In MaterialX 1.38.8 the OSL shader generator will create struct texturesource parameters such as :

    textureresource  shader_file = {\"\", \"\"}
    [[
        string widget = \"filename\"
    ]],

In MaterialX 1.38.10 the shader generator will split the textureresource shader in two parameters :

    string shader_file = \"\"
    [[
        string widget = \"filename\"
    ]],
    string shader_file_colorspace = \"\"
    [[
        string widget = \"colorspace\"
    ]],