Fixed bitmap blit/blend in xlib canvas implementation

  It looks like the source coordinates have to be added to the
  destination coordinates in XRenderComposite or they are
  clamped to the upper left corner of the picture.

  Test case made the demo program button a child of the pixmap and the
  cleared area got redrawn with the pixmap off. Works now.
Overhauled usage part of the manual

  - various corrections
  - added child widget chapter