Разобрался.
Плюс нашел вот такой скрипт:
/* Using multiple Groups to clip the displayed Contents */
var bgImage : Texture2D; // background image that is 256 x 32
var fgImage : Texture2D; // foreground image that is 256 x 32
var playerEnergy = 1.0; // a float between 0.0 and 1.0
function OnGUI () {
// Create one Group to contain both images
// Adjust the first 2 coordinates to place it somewhere else on-screen
GUI.BeginGroup (Rect (0,0,256,32));
// Draw the background image
GUI.Box (Rect (0,0,256,32), bgImage);
// Create a second Group which will be clipped
// We want to clip the image and not scale it, which is why we need the second Group
GUI.BeginGroup (Rect (0,0,playerEnergy * 256, 32));
// Draw the foreground image
GUI.Box (Rect (0,0,256,32), fgImage);
// End both Groups
GUI.EndGroup ();
GUI.EndGroup ();
}