Share bitmaps in addition to sharing observables
Share bitmaps in addition to sharing observables

BitmapDrawable objects can be shared and reused. Sharing drawables
ensures that we load the resources only once, but when drawing the
result we can as well share the drawables.

Part of work on #4481.

ThibaultR/cgeo,mucek4/cgeo,madankb/cgeo,brok85/cgeo,tobiasge/cgeo,rsudev/c-geo-opensource,cgeo/cgeo,Huertix/cgeo,vishwakulkarni/cgeo,Bananeweizen/cgeo,matej116/cgeo,superspindel/cgeo,Huertix/cgeo,tobiasge/cgeo,schwabe/cgeo,KublaikhanGeek/cgeo,matej116/cgeo,SammysHP/cgeo,SammysHP/cgeo,kumy/cgeo,Bananeweizen/cgeo,ThibaultR/cgeo,Bananeweizen/cgeo,superspindel/cgeo,auricgoldfinger/cgeo,samueltardieu/cgeo,KublaikhanGeek/cgeo,marco-dev/c-geo-opensource,rsudev/c-geo-opensource,marco-dev/c-geo-opensource,cgeo/cgeo,cgeo/cgeo,schwabe/cgeo,mucek4/cgeo,S-Bartfast/cgeo,S-Bartfast/cgeo,auricgoldfinger/cgeo,xiaoyanit/cgeo,pstorch/cgeo,kumy/cgeo,pstorch/cgeo,SammysHP/cgeo,mucek4/cgeo,S-Bartfast/cgeo,lewurm/cgeo,marco-dev/c-geo-opensource,rsudev/c-geo-opensource,yummy222/cgeo,lewurm/cgeo,KublaikhanGeek/cgeo,vishwakulkarni/cgeo,samueltardieu/cgeo,xiaoyanit/cgeo,madankb/cgeo,madankb/cgeo,auricgoldfinger/cgeo,superspindel/cgeo,cgeo/cgeo,xiaoyanit/cgeo,schwabe/cgeo,brok85/cgeo,samueltardieu/cgeo,pstorch/cgeo,schwabe/cgeo,kumy/cgeo,brok85/cgeo,yummy222/cgeo,ThibaultR/cgeo,matej116/cgeo,tobiasge/cgeo,vishwakulkarni/cgeo,lewurm/cgeo,Huertix/cgeo,yummy222/cgeo