[BUGFIX] Install Tool image tests: black background after scaling

Transparent turns black unless you specify a different color
because transparent = unset color = 0 = black. We should fix this
by adding the "-opaque white -background white -flatten"
parameters for the convert action done
by the install tool.

Change-Id: I5dd5c41852a33159752702b68ce0a297b33cf569
Releases: 6.3, 6.2
Fixes: #60570
Reviewed-on: http://review.typo3.org/31827
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>