fillLineGap doesn't always fill the gap #253
Description
I've notice a few very small linegaps between row of subtitles when the fiilLineGap option is used.
It is easier to see on content with a light background, but it can be seen if you look carefully with the BBC R&D test stream which is already in the DASH reference player.
stream: https://rdmedia.bbc.co.uk/testcard/simulcast/manifests/avc-ctv.mpd
Attached are output on a window PC running full screen on a 4K monitor for both Chrome and Firefox, both show a linegap, and firefox is wrapping first line of subtitles for some reason.
The linegap is most easily seen in the space between Musical and Scale where the yellow floodlight can be seen, or by zooming right in.
On the firefox image, the linegap can be seen between the two musical notes.
The TTML similar to this
<?xml version="1.0" encoding="utf-8"?>
<tt xmlns="http://www.w3.org/ns/ttml" xmlns:ttp="http://www.w3.org/ns/ttml#parameter" xmlns:tts="http://www.w3.org/ns/ttml#styling" xmlns:ttm="http://www.w3.org/ns/ttml#metadata" xmlns:ebuttm="urn:ebu:tt:metadata" xmlns:ebutts="urn:ebu:tt:style" xmlns:itts="http://www.w3.org/ns/ttml/profile/imsc1#styling" xmlns:ittp="http://www.w3.org/ns/ttml/profile/imsc1#parameter" xml:lang="en" xml:space="default" ttp:timeBase="media" ttp:cellResolution="40 24" ittp:activeArea="33.500000% 7.500000% 33.000000% 60.500000%" >
<head>
<ttm:copyright>British Broadcasting Corporation</ttm:copyright>
<metadata>
<ttm:title>BBC DASH Testcard Stream</ttm:title>
<ebuttm:documentMetadata>
<ebuttm:conformsToStandard>urn:ebu:tt:distribution:2018-04</ebuttm:conformsToStandard>
<ebuttm:conformsToStandard>http://www.w3.org/ns/ttml/profile/imsc1/text</ebuttm:conformsToStandard>
<ebuttm:authoredFrameRate>25</ebuttm:authoredFrameRate>
<ebuttm:documentOriginatingSystem>Perrott Subtitle Encoding</ebuttm:documentOriginatingSystem>
</ebuttm:documentMetadata>
</metadata>
<styling>
<style xml:id="S0" tts:wrapOption="wrap" itts:fillLineGap="true" tts:fontFamily="ReithSans, Arial, Roboto, proportionalSansSerif, default" ebutts:linePadding="0.5c" tts:textAlign="center" tts:fontSize="80%"/>
<style xml:id="S1" tts:textAlign="start"/>
<style xml:id="S10" tts:color="#FFFFFF" tts:fontSize="200%" tts:backgroundColor="#000000"/>
<style xml:id="S11" tts:color="#FFFF00" tts:fontSize="200%" tts:backgroundColor="#000000"/>
<style xml:id="S12" tts:color="#FF9010" tts:fontSize="200%" tts:backgroundColor="#000000"/>
<style xml:id="S16" tts:fontStyle="italic" tts:color="#10FF10" tts:fontSize="200%" tts:backgroundColor="#000000"/>
</styling>
<layout>
<region xml:id="R3" tts:origin="33.5% 32%" tts:extent="33% 36%" tts:overflow="visible" tts:displayAlign="before"/>
<region xml:id="R8" tts:origin="35% 7.5%" tts:extent="30% 10%" tts:overflow="hidden" tts:displayAlign="before"/>
</layout>
</head>
<body style="S0">
<div region="R8">
<p xml:id="p8-1-470" begin="473631:40:40.32" end="473631:40:44.16"><span style="S16">Top centre</span></p>
</div>
<div region="R3">
<p xml:id="p3-6-470" style="S1"><span style="S10" begin="473631:40:40.32" end="473631:40:44.16">♫ Musical Scale ♪<br/></span><span style="S12" begin="473631:40:40.32" end="473631:40:44.16">C</span><span style="S12" begin="473631:40:40.32" end="473631:40:44.16">, D</span><span style="S12" begin="473631:40:40.32" end="473631:40:44.16">, E</span><span style="S12" begin="473631:40:40.32" end="473631:40:44.16">, F</span><span style="S12" begin="473631:40:40.32" end="473631:40:44.16">, G</span><span style="S12" begin="473631:40:40.5" end="473631:40:44.16">, A</span><span style="S12" begin="473631:40:40.7" end="473631:40:44.16">, B</span><span style="S12" begin="473631:40:40.9" end="473631:40:44.16">, C</span><span style="S12" begin="473631:40:41.1" end="473631:40:44.16">, D</span><span style="S12" begin="473631:40:41.3" end="473631:40:44.16">, E</span><span style="S12" begin="473631:40:41.5" end="473631:40:44.16">, F</span><span style="S12" begin="473631:40:41.7" end="473631:40:44.16">, G</span><span style="S12" begin="473631:40:41.9" end="473631:40:44.16">, A</span><span style="S12" begin="473631:40:42.1" end="473631:40:44.16">, B</span><span style="S11" begin="473631:40:42.3" end="473631:40:44.16">, C</span><span style="S11" begin="473631:40:42.5" end="473631:40:44.16">, B</span><span style="S11" begin="473631:40:42.7" end="473631:40:44.16">, A</span><span style="S11" begin="473631:40:42.9" end="473631:40:44.16">, G</span><span style="S11" begin="473631:40:43.1" end="473631:40:44.16">, F</span><span style="S11" begin="473631:40:43.3" end="473631:40:44.16">, E</span><span style="S11" begin="473631:40:43.5" end="473631:40:44.16">, D</span><span style="S11" begin="473631:40:43.7" end="473631:40:44.16">, C</span><span style="S11" begin="473631:40:43.9" end="473631:40:44.16">, B</span><span style="S11" begin="473631:40:44.1" end="473631:40:44.16">, A</span></p>
</div>
</body>
</tt>