Comment 47 for bug 248271

We don't just flip between images when they have alpha; we have to do some compositing. It seems like doing a SOURCE would be optimal here, but we still do a clear on source - http://hg.mozilla.org/mozilla-central/file/1b724a06a345/modules/libpr0n/src/imgContainer.cpp#l1454 - I will file a followup bug on whether we can optimize that to OPERATOR_SOURCE. I think yes, but if we're drawing directly to the destination, there might be weirdness.

We always decode to RGB(A). We only get benefits from that if we're decoding RGB-only images, because we can use OPERATOR_SOURCE, which saves a read.

(Incidentally, inspecting imgContainer for commenting on this bug led to me finding bug 513749, which I'm glad to have found!)