glGetFragDataLocation - query the bindings of color numbers to user-defined varying out variables
GLint glGetFragDataLocation(GLuint program, const char * name);
glGetFragDataLocation retrieves the assigned color number binding for the user-defined varying out variable name for program program. program must have previously been linked. name must be a null-terminated string. If name is not the name of an active user-defined varying out fragment shader variable within program, -1 will be returned.
GL_INVALID_OPERATION is generated if program is not the name of a program object.
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