allow image widgets to do alpha-blitting
allow image widgets to do alpha-blitting

when the image (e.g. a PNG) does contain an alpha channel, that can
be honored by ImageWidget. It doesn't do so by default for compatibility
(arguably, we should change that in the future), it has to be enabled
by setting the "alpha" property to "true" (boolean, not string).

NiLuJe/koreader,poire-z/koreader,houqp/koreader,Frenzie/koreader,koreader/koreader,pazos/koreader,NiLuJe/koreader,koreader/koreader,ashang/koreader,chihyang/koreader,mihailim/koreader,NickSavage/koreader,noname007/koreader,mwoz123/koreader,lgeek/koreader,chrox/koreader,Frenzie/koreader,ashhher3/koreader,robert00s/koreader,apletnev/koreader,frankyifei/koreader,Hzj-jie/koreader,Markismus/koreader,poire-z/koreader