So that similar to ODT, images are not loaded on file open, only when
the user scrolls there.

Notes:

1) GraphicDescriptor::ImpDetectJPG() would try to calculate the logic
size before the pixel size is available, so the logic size would be 0x0.
Also, ImpGraphic::ImplSetPrepared() would always work with a pixel map
mode. Any of these two would result in a failure of
testDMLShapeFillBitmapCrop in CppunitTest_sw_ooxmlexport6.

2) Lazy-loading seems to (at the moment) not recognize EMF files, so
don't lazy-load in case an external header is provided. This probably
has to be revisited, since the ODF import doesn't go via
GraphicProvider::queryGraphic().

Change-Id: I44754e659effebca8339715df114dbaadb9b5e9f
Reviewed-on: https://gerrit.libreoffice.org/53215Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>