[BUGFIX] Source element within picture element has wrong src attribute

Change it from src to srcset as described in the latest HTML5
specification for master and 6.2

Resolves: #63107
Releases: master, 6.2
Change-Id: Ia3180873af948db30bb60f2f071e5ccfe2283baf
Reviewed-on: http://review.typo3.org/34585
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>