ttmlparse: fix issues in aggregation of input TTML

The last commit to ttmlparse (!848 (merged)) broke the handling of TTML that is delivered as DASH segments. This can be demonstrated with the following command:

GST_TTML_AUTOPLUG=1 gst-play-1.0 http://rdmedia.bbc.co.uk/dash/ondemand/elephants_dream/1/client_manifest-one_representation_per_component.mpd

The reason is that the TTML documents within the ISOBMFF segments contain a linefeed (0xA) after the closing </tt> tag. !848 (merged) searches for this closing tag to find whether a complete TTML file has been received and where it ends in the input; however, any trailing whitespace is then added to the start of the next TTML file to be parsed, which causes libxml2 to fail (it expects files to start with an XML declaration).

Additionally, the code in !848 (merged) searches the aggregated input for the last </tt> tag. It should be looking for the first </tt> tag, as there may be multiple complete TTML documents in the aggregated input, and the parser can handle only one TTML document at a time.

The commits in this MR fix these issues.

looks good to me

added Regression label

@ao2 do you have any chance to check that this wouldn't break your use case?

resolved all threads

assigned to @gstreamer-merge-bot

added 1.18 Maybe backport labels

mentioned in commit floobleflam/gst-plugins-bad@0db656c0

mentioned in commit floobleflam/gst-plugins-bad@9233cc19

added 269 commits

• 41677c47...bfb90710 - 267 commits from branch gstreamer:master
• 9233cc19 - ttmlparse: Ensure only single TTML doc parsed
• 0db656c0 - ttmlparse: Handle whitespace before XML declaration

Compare with previous version

assigned to @floobleflam and unassigned @gstreamer-merge-bot

I couldn't merge this branch: CI failed!

assigned to @gstreamer-merge-bot and unassigned @floobleflam

mentioned in commit floobleflam/gst-plugins-bad@8df2314c

mentioned in commit floobleflam/gst-plugins-bad@d25be0d1

added 3 commits

• dd7b6728 - 1 commit from branch gstreamer:master
• 8df2314c - ttmlparse: Ensure only single TTML doc parsed
• d25be0d1 - ttmlparse: Handle whitespace before XML declaration

Compare with previous version

merged

removed Maybe backport label

added Needs backport label

mentioned in commit tpm/gst-plugins-bad@2d362151

mentioned in commit tpm/gst-plugins-bad@89bf5622