Encode paletted PNGs more efficiently

Saves about 2 MB of encoded size across affected assets.

Also will enable more efficient decoding.

Specifically, encoded palette values are assumed to be opaque unless
alpha values are provided in a tRNS chunk.  Before this change, we
would wastefully store many opaque alpha values in tRNS chunk.
Additionally, the decoder used to need to premultiply all of these
opaque colors, because the encoded data indicated that they had alpha.

Change-Id: I1d5400f71a0e3cb800fd0fc68a04c8c4069da8ef