mapClip Library Method
Short Description: Clip mapped texture
Signature: t.mapClip (w,h,pos,overflow)
Class: transition Class
Uses a rectangular portion of the currently mapped texture as texture for the material.
w is the with and h the height of the portion. pos may be "tr" for the top-right,
"tl" for the top-left, "br" for the bottom-right, and "bl" for bottom-left portion.
pos can also be "t" for the top, "b" for the bottom, "l" for the left, and "r" for the right portion.
Normally w and h are automatically clipped to the size of the current portion.
This behaviour can be disabled using overflow="b" for h, overflow="r" for w and
overflow="br" for both w and h.
mapClip is similar to map with the difference that the coordinates are
determinated automatically based on pos and overflow. See also mapClipToElement for