fix #4481: cache rendering leads to ANR
fix #4481: cache rendering leads to ANR

This fix is only partially correct, in that it will not recompute the
whole layout (including the position of line breaks) when images are
loaded.

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