[BUGFIX] Set currentVal in 2nd rendering loop of IMGTEXT

Set the currentVal to the current filename during rendering of
HTML in the second rendering loop. This makes current=1 useful for
rendering captions.

Change-Id: I335bb2f744f9352428f6b0d1931ccf2d80f45bb6
Fixes: #28794
Releases: 4.7, 4.6, 4.5
Reviewed-on: http://review.typo3.org/6676
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter