cwebp (windows): fix alpha image import on XP
cwebp (windows): fix alpha image import on XP

Query the converter to ensure the format is supported; add BGR formats
as RGBA was failing for PNG on XP

Fixes issue 129

Change-Id: I02e0d74b3b21337bc5fffd6a5dc158b7809b9aa9

appunite/libwebp,ericmckean/webm.libwebp,kim42083/webm.libwebp,Tekserve/libwebp,zofuthan/libwebp,Distrotech/libwebp,Distrotech/libwebp,appunite/libwebp,Aexyn/libwebp,iniwf/webm.libwebp,keane83/BLE_IM,appunite/libwebp,ttyangf/libwebp,matanbs/webm.libwebp,kim42083/webm.libwebp,imazen/libwebp,kalli123/webm.libwebp,kalli123/webm.libwebp,keane83/BLE_IM,Tekserve/libwebp,reimaginemedia/webm.libwebp,Suvarna1488/webm.libwebp,Distrotech/libwebp,redengineer/libwebp,jacklicn/libwebp,PKRoma/libwebp,dumganhar/libwebp,ttyangf/libwebp,ttyangf/libwebp,yohunl/libwebp,abwiz0086/webm.libwebp,matanbs/webm.libwebp,yohunl/libwebp,cysp/libwebp,comoyo/gonzo-libwebp,Acidburn0zzz/webm.libwebp,reimaginemedia/webm.libwebp,comoyo/gonzo-libwebp,comoyo/gonzo-libwebp,Aexyn/libwebp,jacklicn/libwebp,comoyo/gonzo-libwebp,iniwf/webm.libwebp,imazen/libwebp,abwiz0086/webm.libwebp,rxl194/libwebp,Tekserve/libwebp,jacklicn/libwebp,imazen/libwebp,Aexyn/libwebp,jiehu5114/libwebp,zofuthan/libwebp,rxl194/libwebp,kalli123/webm.libwebp,Maria1099/webm.libwebp,Maria1099/webm.libwebp,gshORTON/webm.libwebp,ya7lelkom/libwebp,appunite/libwebp,jacklicn/libwebp,Aexyn/libwebp,rxl194/libwebp,gshORTON/webm.libwebp,comoyo/gonzo-libwebp,ttyangf/libwebp,gshORTON/webm.libwebp,yohunl/libwebp,PKRoma/libwebp,imazen/libwebp,comoyo/gonzo-libwebp,gshORTON/webm.libwebp,cysp/libwebp,Maria1099/webm.libwebp,reimaginemedia/webm.libwebp,Suvarna1488/webm.libwebp,abwiz0086/webm.libwebp,altogother/webm.libwebp,ericmckean/webm.libwebp,ericmckean/webm.libwebp,altogother/webm.libwebp,abwiz0086/webm.libwebp,webmproject/libwebp,altogother/webm.libwebp,comoyo/gonzo-libwebp,redengineer/libwebp,Suvarna1488/webm.libwebp,Suvarna1488/webm.libwebp,jiehu5114/libwebp,ericmckean/webm.libwebp,rxl194/libwebp,reimaginemedia/webm.libwebp,kleopatra999/webm.libwebp,gshORTON/webm.libwebp,jiehu5114/libwebp,cysp/libwebp,appunite/libwebp,Acidburn0zzz/webm.libwebp,Tekserve/libwebp,zofuthan/libwebp,iniwf/webm.libwebp,Acidburn0zzz/webm.libwebp,iniwf/webm.libwebp,webmproject/libwebp,yohunl/libwebp,abwiz0086/webm.libwebp,Aexyn/libwebp,Acidburn0zzz/webm.libwebp,kleopatra999/webm.libwebp,dumganhar/libwebp,webmproject/libwebp,kim42083/webm.libwebp,cysp/libwebp,redengineer/libwebp,altogother/webm.libwebp,ya7lelkom/libwebp,zofuthan/libwebp,imazen/libwebp,kleopatra999/webm.libwebp,PKRoma/libwebp,Maria1099/webm.libwebp,zofuthan/libwebp,dumganhar/libwebp,yohunl/libwebp,ttyangf/libwebp,ya7lelkom/libwebp,PKRoma/libwebp,reimaginemedia/webm.libwebp,matanbs/webm.libwebp,matanbs/webm.libwebp,webmproject/libwebp,matanbs/webm.libwebp,jiehu5114/libwebp,PKRoma/libwebp,kalli123/webm.libwebp,rxl194/libwebp,jiehu5114/libwebp,Suvarna1488/webm.libwebp,altogother/webm.libwebp,kleopatra999/webm.libwebp,redengineer/libwebp,Maria1099/webm.libwebp,ya7lelkom/libwebp,PKRoma/libwebp,appunite/libwebp,keane83/BLE_IM,redengineer/libwebp,ericmckean/webm.libwebp,keane83/BLE_IM,jacklicn/libwebp,keane83/BLE_IM,ya7lelkom/libwebp,kleopatra999/webm.libwebp,kim42083/webm.libwebp,kalli123/webm.libwebp,kim42083/webm.libwebp,iniwf/webm.libwebp,webmproject/libwebp,Acidburn0zzz/webm.libwebp