dwebp/WritePNG: mark png variables volatile
dwebp/WritePNG: mark png variables volatile

these are used on both sides of the setjmp().

Change-Id: I4a789bfb3a5d56946a22286c5a140008d90e1ba2

imazen/libwebp,redengineer/libwebp,jiehu5114/libwebp,zofuthan/libwebp,webmproject/libwebp,kleopatra999/webm.libwebp,jacklicn/libwebp,yohunl/libwebp,ya7lelkom/libwebp,matanbs/webm.libwebp,kleopatra999/webm.libwebp,webmproject/libwebp,matanbs/webm.libwebp,jacklicn/libwebp,webmproject/libwebp,altogother/webm.libwebp,reimaginemedia/webm.libwebp,ericmckean/webm.libwebp,Acidburn0zzz/webm.libwebp,kalli123/webm.libwebp,jiehu5114/libwebp,reimaginemedia/webm.libwebp,ttyangf/libwebp,ttyangf/libwebp,yohunl/libwebp,ericmckean/webm.libwebp,yohunl/libwebp,rxl194/libwebp,altogother/webm.libwebp,abwiz0086/webm.libwebp,altogother/webm.libwebp,Suvarna1488/webm.libwebp,gshORTON/webm.libwebp,ya7lelkom/libwebp,zofuthan/libwebp,reimaginemedia/webm.libwebp,Suvarna1488/webm.libwebp,Aexyn/libwebp,zofuthan/libwebp,matanbs/webm.libwebp,abwiz0086/webm.libwebp,kim42083/webm.libwebp,altogother/webm.libwebp,ttyangf/libwebp,zofuthan/libwebp,PKRoma/libwebp,Acidburn0zzz/webm.libwebp,ya7lelkom/libwebp,kleopatra999/webm.libwebp,Suvarna1488/webm.libwebp,matanbs/webm.libwebp,reimaginemedia/webm.libwebp,Maria1099/webm.libwebp,PKRoma/libwebp,imazen/libwebp,redengineer/libwebp,kim42083/webm.libwebp,ttyangf/libwebp,webmproject/libwebp,jiehu5114/libwebp,iniwf/webm.libwebp,altogother/webm.libwebp,Maria1099/webm.libwebp,yohunl/libwebp,rxl194/libwebp,rxl194/libwebp,ya7lelkom/libwebp,gshORTON/webm.libwebp,Acidburn0zzz/webm.libwebp,ericmckean/webm.libwebp,webmproject/libwebp,jiehu5114/libwebp,imazen/libwebp,redengineer/libwebp,ttyangf/libwebp,Maria1099/webm.libwebp,kleopatra999/webm.libwebp,kleopatra999/webm.libwebp,rxl194/libwebp,iniwf/webm.libwebp,Aexyn/libwebp,kim42083/webm.libwebp,kalli123/webm.libwebp,Maria1099/webm.libwebp,iniwf/webm.libwebp,kim42083/webm.libwebp,Aexyn/libwebp,matanbs/webm.libwebp,Aexyn/libwebp,jiehu5114/libwebp,gshORTON/webm.libwebp,PKRoma/libwebp,Acidburn0zzz/webm.libwebp,PKRoma/libwebp,jacklicn/libwebp,redengineer/libwebp,zofuthan/libwebp,Aexyn/libwebp,kalli123/webm.libwebp,ericmckean/webm.libwebp,Acidburn0zzz/webm.libwebp,abwiz0086/webm.libwebp,PKRoma/libwebp,abwiz0086/webm.libwebp,yohunl/libwebp,iniwf/webm.libwebp,imazen/libwebp,reimaginemedia/webm.libwebp,imazen/libwebp,PKRoma/libwebp,ericmckean/webm.libwebp,jacklicn/libwebp,gshORTON/webm.libwebp,gshORTON/webm.libwebp,Suvarna1488/webm.libwebp,Maria1099/webm.libwebp,redengineer/libwebp,abwiz0086/webm.libwebp,rxl194/libwebp,iniwf/webm.libwebp,kalli123/webm.libwebp,jacklicn/libwebp,ya7lelkom/libwebp,Suvarna1488/webm.libwebp,kalli123/webm.libwebp,kim42083/webm.libwebp