in GetMsgPanelInfo() (PPI, scale).
m_image name was confusing: it is not a wxImage, but a BITMAP_BASE item,
and m_image name was already in use in BITMAP_BASE, used inside SCH_IMAGE.
Bitmap properties dialog: show PPI value and add Title to dialog.
Also adds some debugging to try to catch dangling pointers.
Also adds a cache for group bounding boxes (which will be expensive
to calculate for large groups).
Fixes https://gitlab.com/kicad/code/kicad/issues/12875
jean-pierre charras
Jon Evans
Jeff Young
Mike Williams