better graphical preparation (currently BMP_SCALE_BESTQUALITY, but can
be changed in a single place now if wanted)

(cherry picked from commit d32f1d35)

Conflicts:
	svx/inc/svx/galtheme.hxx
	svx/source/gallery2/galctrl.cxx
	svx/source/gallery2/galtheme.cxx

Change-Id: I80879ca472c784f764126676046c1388e1167652