Quad:setViewport
Sets the texture coordinates according to a viewport.
Quad:setViewport( x, y, w, h, sw, sh )
x number The top-left corner along the x-axis.
y number The top-left corner along the y-axis.
w number The width of the viewport.
h number The height of the viewport.
sw number The reference width, the width of the Image. (Must be greater than 0.)
sh number The reference height, the height of the Image. (Must be greater than 0.)