Magic number elimination #1: Remove the 100px default for mirror width and height.
Magic number elimination #1:
Remove the 100px default for mirror width and height.

Old algorithm was:
  * Use cached sheet-mirror-region if present.
  * Else use keyword argument if given.
  * Else use default argument value 100.

(The third code path was actually being used.)

New algorithm:
  * Use keyword argument if given.
  * Else use cached sheet-mirror-region.

informatimago/mcclim,informatimago/mcclim,quek/mcclim,quek/mcclim