Highlight error lines in minimap

[miv391]

Implements this enhancement: godotengine/godot-proposals#8444

Error lines (= lines with non-default background color) are using alpha = 1.0 instead of alpha *= 0.6 as other lines, thus making the error lines more brighter.

Production edit: Closes godotengine/godot-proposals#8444

[MewPurPur]

I don't think this needs to be an editor setting, after all, we already highlight the current line and don't have a setting to remove that.

[miv391]

I removed the editor setting for highlighting the error lines as requested.

[Calinou]

Tested locally, it works as expected:

[YuriSizov]

Thanks!