Allow materials to specify default values for properties

[randomPoison]

Currently material properties default to the default value for their type. While this is sometimes fine, there are cases where the default value doesn't work or their is a more reasonable default value for the property. For these cases it would be better if an alternate default value could be specified in the shader, e.g.:

property surface_color: Color = (1.0, 0.0, 0.0, 1.0);
poperty shininess: f32 = 4.0;