[Swftools-common] PNG alpha

[Andrew Wason]

I noticed this comment in swftools-0.5.1/lib/modules/swfbits.c:

      Indeed: I haven't understood yet how flash player handles
      alpha values different from 0 and 0xff in lossless bitmaps...

I think the flash player expects the alpha values to be premultiplied 
(for both indexed and 32bit ARGB). So for a pixel A R G B, store
A A/255*R A/255*G A/255*B

Andrew