Fix explicit template instantiation

[DanRStevens]

It seems explicit template instantiation was broken for Point and Rectangle, and doesn't exist for Vector.

The extern template declarations were accidentally removed for Point and Rectangle. It seems they were confused with the type aliases to the old non-template struct definitions that used to exist.

Added:

• commit 0d1af6d (Rectangle) (PR Rectangle template #345)
• commit 9d3f774 (Point) (PR Point template #346)

Removed:

• commit fe1b5bf (Point) (PR Remove Point and Rectangle type aliases #654)
• commit b9a8855 (Rectangle) (PR Remove Point and Rectangle type aliases #654)

We should fix Point and Rectangle, and add the missing explicit instantiation for Vector.

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We should also consider adding explicit instantiation for the Signal types used in EventHandler.

Related:

• Issue Improve build times OutpostUniverse/OPHD#1573

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Documentation:
https://en.cppreference.com/w/cpp/language/class_template