Horde3D

#define DEG2RAD(x)  (((x)/180)*3.14159265)

typedef struct {
   float x,y;
} point;

point ll = {0.0, 0.0};
point lr = {0.1, 0.0};
point ur = {0.1, 0.1};
point ul = {0.0, 0.1};

point center;
center.x = (ll.x + lr.x) / 2.0;
center.y = (ll.x + ul.y) / 2.0;

float rot = DEG2RAD(45); // rotate 45 degrees

void rotate_2d(point &p) {
   float newx = (cos(rot) * (p.x - center.x)) + (sin(rot) * (p.y - center.y));
   float newy = (-sin(rot) * (p.x - center.x)) + (cos(rot) * (p.y - center.y));

   p.x = newx + center.x;
   p.y = newx + center.y;
}

rotate_2d(ll);
rotate_2d(lr);
rotate_2d(ur);
rotate_2d(ul);

Horde3D::showOverlay(ll.x, ll.y, 0, 0, lr.x, lr.y, 1, 0, ur.x, ur.y, 1, 1, ul.x, ul.y, 0, 1, 1, res);