fix:GML2:readFlatCoordinates can return the format of (x,y) or (y,x)

[jacklinna]

Fixes #15617

[github-actions]

📦 Preview the website for this branch here: https://deploy-preview-15675--ol-site.netlify.app/.

[ahocevar]

Thanks, @jacklinna. You'd also have to update the tests.

[ahocevar]

Also, the failing tests show that the fix might not be as straightforward as you thought. This one for example:

Chrome Headless 123.0.0.0 (Linux x86_64) ol.format.WFS when parsing mapserver GML2 polygon creates a polygon for My Polygon with hole FAILED
	Error: expected [ 47.003018, -0.768746, 0.532597, undefined ] to sort of equal [ 47.003018, -0.768746, 47.925567, 0.532597 ]
	    at Assertion.assert (/home/runner/work/openlayers/openlayers/node_modules/expect.js/index.js:96:13)
	    at Assertion.eql (/home/runner/work/openlayers/openlayers/node_modules/expect.js/index.js:230:10)
	    at Context.<anonymous> (/tmp/_karma_webpack_683077/webpack:/ol/test/browser/spec/ol/format/wfs.test.js:141:1)