WebPEncode: Support encoding same pic twice (even if modified)

This wasn't working for this specific scenario:
- Encode an RGBA 'pic' (with trivial alpha) using lossy encoding.
(so that pic->a == NULL after import happens).
- Modify the 'pic->argb' so that it has non-trivial alpha.
- Encode the same 'pic' again.
This used to fail to encode alpha data as pic->a == NULL.

Change-Id: Ieaaa7bd09825c42f54fbd99e6781d98f0b19cc0c
Merge "WebPEncode: Support encoding same pic twice (even if modified)"