Both Readers : Read a line back, and write one that runs right to left the right way round - #988

Description

Both Writers write a Line and neither Reader read one back, so a deck that was loaded and saved again lost every line it had. Reader/PowerPoint2007::loadSlideShapes() switches on the tag name and had no p:cxnSp arm; Reader/ODPresentation::loadSlide() walked draw:frame only, and a draw:line is a shape of the page rather than the content of a frame.

Measuring the round trip turned up a second defect, in the Writer, which had to go with it. An OOXML extent cannot be negative, so a line whose end is behind its start is written from the other end with flipH / flipV saying which axis it is mirrored on. writeShapeLine() wrote flipV for a line that runs right to left, and nothing at all for one that runs up and to the left:

So a line drawn right to left arrived mirrored in PowerPoint, with no reader involved. PptSlidesTest::testLine created all four lines but asserted only that some a:xfrm carried a flipV, which the second case satisfies on its own, so nothing caught it. That test now says which of the three forms it expects, and the two readers each round-trip a line in all four directions.

The ODPresentation Writer needed nothing: ODF says a line by the two points it runs between, so a negative width is simply an svg:x2 smaller than the svg:x1.

Fixes #987

Checklist:

• I have run composer run-script check --timeout=0 and no errors were reported

phpunit, phpstan, phpmd and php-cs-fixer all clean.

• The new code is covered by unit tests (check build & code coverage report)

testLineSurvivesTheRoundTrip in both Reader tests, over the four directions a line can run, and the three flip forms asserted in PptSlidesTest::testLine.

• I have updated the documentation to describe the changes

Nothing to describe: createLineShape() is documented already and its behaviour is unchanged.

• I have added an entry in the CHANGELOG.md file

Under Bug fixes.