Preserve background color with RGB PNG export
So far, background color is preserved in non-alpha-channel bitmap exports (e.g. PNG RGB, JPEG, ...) if the document background color alpha is between 1 and 255. But for alpha=0, it gets lost because alpha is premultiplied during rendering, resulting in black background color. This patch fixes the issue.
Fixes #1357 (closed) Fixes extensions#366 (closed) Fixes inbox#5743 (closed)