Save Image-Crop information in docx for PNG

 Issue:-
  1] When MS Office docx file containing cropped image is
     round tripped with LibreOffice 4.2, image looses
     its cropping effect.

 Implementation:-
  1] Function WriteSrcRect() is implemented inside docxattributeoutput.cxx
  2] It writes out xml tag <a:srcRect> which is responsible for storing
     cropping parameters

Change-Id: I8814df40f9907512779b42c9ad2405953c70626b
Reviewed-on: https://gerrit.libreoffice.org/6507
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>