PicLayer plugin should allow load calibration with scale only

[vadim@…]

When using PicLayer, in many cases there are multiple images with same scale, but different positions. However PicLayer only allows to load complete calibration with position of image, which is inconvenient, as you need to move layer over long distance every time. It'd be nice to allow load calibration when only: SCALEY,SCALEX, and ANGLE are set up. Other parameters if not defined, should be left as is (position, scale, etc). Currently it fails with "Loading file failed: null" when any of parameters is not defined.