configure: move config.h to src/webp/config.h

this change has the side-effect of using directory names in the
include, silencing a lint warning.

Change-Id: Ib91cf63a90534e32fadfa5c2372bfdb29f854d02
properly report back encoding error code in WebPFrameCacheAddFrame()

User-hook can fail but error was not propagated back.

Change-Id: Ic79f9543bf767634a127eccfef90af855ff15c34
Also: some ad-hoc clean-up and API dusting. More to come later...
Merge "configure: move config.h to src/webp/config.h"