fix const cy::TriMesh::Str to const char * conversion

[linesight]

bool TextureManager::loadTexture(const cy::TriMesh::Str& filename, cy::GLTexture2D& texture)
{
    unsigned char* decode = 0;
    unsigned width, height;
    unsigned int error = lodepng_decode24_file(&decode, &width, &height, filename);
}

would throw error 'unsigned int lodepng_decode24_file(unsigned char **,unsigned int *,unsigned int *,const char *)': cannot convert argument 4 from 'const cy::TriMesh::Str' to 'const char *'