[expression] Negative index support to point_n(), angle_at_vertex(), distance_to_vertex() functions

[nirvn]

Description

Because we all love negative index support.

Checklist

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[nyalldawson]

Can you update the help accordingly?

[nirvn]

@nyalldawson , that's not a bad idea! 😉

[DelazJ]

@nirvn see https://issues.qgis.org/issues/21284 (and related https://issues.qgis.org/issues/21282)

[nirvn]

@nyalldawson , function help ammended.

@DelazJ , negative index support added to angle_at_vertex() and distance_to_vertex() functions. We can't harmonize the index (1 vs 0 as starting point) until QGIS 4.0, let's keep that in mind when we cross that bridge.