Extend NavigationMeshSourceGeometryData[23]D to allow data merging

[Scony]

This PR adds merge method to NavigationMeshSourceGeometryData3D

This PR therefore adds a way to avoid boilerplate code such as lampe-games/godot-open-rts@487e995#diff-0291eaa88b59eec77f3d1546e6404bea1391d58eec1ee646874dded17f0bfe16R60-R71

[smix8]

Imo it would be better to add a merge() function on the source geometry resource with another source geometry resource as the parameter, e.g. like Dictionary does it.

Just because you want to merge something does not mean you want to have all the overhead of the single-threaded parsing again as well. Many users create their entire source geometry procedual in scripts never using the parse function. They might still want the ability to merge two of them together which sounds useful doing in C++ compared to doing a slow copy in scripts.

Also changing the parse function on the NavigationServer breaks the function signature and compatibility while adding a merge() function does not.

[smix8]

Approve for the API.

Not sure about the code style, the use of ::Size looks exotic compared to the rest of the engine.

[Mickeon]

Seems like the "Size" alias is a relatively new thing since the 64-bit CowData upgrade. A quick look-up tells me it has never been used like this before. Other code just... puts the size into an int64_t or int32_tfor simplicity, although this is more future-proof.

Personally I would just do that, int64_t for consistency. Alternatively it should be better as a global Size alias but that would be a change to apply across the whole codebase...?

[akien-mga]

Thanks!