Fix incorrect area calculation in QgsLineString::sumUpArea

[lbartoletti]

Description

This PR fixes the area calculation in the QgsLineString::sumUpArea method. The previous formula incorrectly computed the area for closed rings, always returning negative values regardless of the orientation of the vertices. The issue occurred because the formula used in the loop didn't properly account for the orientation of the line.

Changes:

• Updated the area calculation using the correct shoelace formula, ensuring that areas for clockwise-oriented rings return negative values, and counterclockwise-oriented rings return positive values.
• Added a check to ensure the method only calculates the area for polygons with at least 3 points.

This resolves the issue returned by @agiudiceandrea where both clockwise and counterclockwise rings returned negative areas.

Fixes #58333

⚠️ Since I also "fix" tests, let's open it as draft and see how many tests have to be fixed.⚠️

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🪟 Windows builds

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(Built from commit fc2a221)

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Tests failed for Qt 6

One or more tests failed using the build from commit fc2a221

line_offset_negative

line_offset_negative

Test failed at testLineOffset at tests/src/python/test_qgshashlinesymbollayer.py:377

Rendered image did not match tests/testdata/control_images/symbol_hashline/expected_line_offset_negative/expected_line_offset_negative.png (found 4881 pixels different)

lineburst_offset

lineburst_offset

Test failed at testLineOffset at tests/src/python/test_qgslineburstsymbollayer.py:226

Rendered image did not match tests/testdata/control_images/symbol_lineburst/expected_lineburst_offset/expected_lineburst_offset.png (found 5543 pixels different)

rasterline_offset

rasterline_offset

Test failed at testLineOffset at tests/src/python/test_qgsrasterlinesymbollayer.py:178

Rendered image did not match tests/testdata/control_images/symbol_rasterline/expected_rasterline_offset/expected_rasterline_offset.png (found 1166 pixels different)

The full test report (included comparison of rendered vs expected images) can be found here.

Further documentation on the QGIS test infrastructure can be found in the Developer's Guide.

[github-actions]

Tests failed for Qt 5

One or more tests failed using the build from commit fc2a221

embedded_mapinfo_lines

embedded_mapinfo_lines

Test failed at testMapInfoLineSymbolConversion at tests/src/python/test_qgsembeddedsymbolrenderer.py:250

Rendered image did not match tests/testdata/control_images/embedded/expected_embedded_mapinfo_lines/expected_embedded_mapinfo_lines.png (found 24839 pixels different)

line_offset_negative

line_offset_negative

Test failed at testLineOffset at tests/src/python/test_qgshashlinesymbollayer.py:377

Rendered image did not match tests/testdata/control_images/symbol_hashline/expected_line_offset_negative/expected_line_offset_negative.png (found 4841 pixels different)

lineburst_offset

lineburst_offset

Test failed at testLineOffset at tests/src/python/test_qgslineburstsymbollayer.py:226

Rendered image did not match tests/testdata/control_images/symbol_lineburst/expected_lineburst_offset/expected_lineburst_offset.png (found 5543 pixels different)

rasterline_offset

rasterline_offset

Test failed at testLineOffset at tests/src/python/test_qgsrasterlinesymbollayer.py:178

Rendered image did not match tests/testdata/control_images/symbol_rasterline/expected_rasterline_offset/expected_rasterline_offset.png (found 1166 pixels different)

The full test report (included comparison of rendered vs expected images) can be found here.

Further documentation on the QGIS test infrastructure can be found in the Developer's Guide.