tvg_saver: optimize the bitmap data with premultiplied alpha processing.

[hermet]

We observed, one tvg picture has a broken image:

This problem arises because the current raw_loader expects a premultiplied-alpha image, while the tvg saver writes it with a straight-alpha image. We can optimize the tvg saver to support encoding bitmap data with premultiplying. Alternatively, the bitmap image loader could allow an option for 'premultiply' for the bitmap data. Eitherway it could fix this problem but the first approach is more preferred.