[TASK] Enable image configuration png_truecolor by default

Since TYPO3 4.6 ships with the requirement of IM6, the truecolor
processing on PNG files can be enabled by default.

Change-Id: I7be3cceeb92bc7dba6c3d59bec694bb4fc49940b
Resolves: #31159
Reviewed-on: http://review.typo3.org/6224
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers