[Bug]: ScriptingGradient doesnt include alpha

[Alasdairbugs]

SuperTux Version

v0.6.3-1566-ge43838b1d

System Information

win 11 64 bit

Expected Behavior

Gradients in the game have an RGBA component

Actual Behavior

but the scripting infrastructure in the game only accommodates RGB

Steps To Reproduce Actual Behavior

Check the Scripting reference for Scripting Gradient

Additional Information

No response

Guidelines For Reporting Issues

• I have read https://github.com/SuperTux/supertux/blob/master/CONTRIBUTING.md#bug-reports.
• I have verified this isn't an issue that's already been reported.
• I have verified this isn't a discussion, or an issue about a crash or a feature request, but rather an actual bug ─ that is, the game did something not intended.
• I have verified this issue is not about wrong translations (use Transifex for those), or anything unsupported (e.g. third-party add-ons).
• In this report, I have only included details about one (1) bug.
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