Adds a double-sided quad to the scene.
The quad is initially centered at (0, 0, 0). Its normal is the
z axis. The quad itself is
composed of a user-defined number of triangles regularly spaced on a grid. This is the main way
to draw height maps.
w- the quad width.
h- the quad height.
usubdivs- number of horizontal subdivisions. This correspond to the number of squares which will be placed horizontally on each line. Must not be
vsubdivs- number of vertical subdivisions. This correspond to the number of squares which will be placed vertically on each line. Must not be