Don't use constant reference in Vector push_back, insert and append_array

[qarmin]

Fixes #31159 and similar

Test project - Line.zip - To see errors Valgrind or Address Sanitizer should be used

Reduz explanation from #31609 (comment)

Sorry, this should have been fixed long ago, we even discussed it at the beginning of the year.

I suggest just make a PR that changes this:

bool Vector<T>::push_back(const T &p_elem) 
to this
bool Vector<T>::push_back(T p_elem) 

It should be fine performance wise, the rational is that in far most cases (Specially where performance is required), Godot uses types that are 8 bytes or less, which is just the same as a pointer. This is fast because it will fit on a CPU register or cache. For larger datatypes, this is not really something critically performant anyway because a good amount of bytes has to be copied or a copy constructor needs to be run, so it should not matter much if it does a copy.

[qarmin]

I changed file editor/import/editor_scene_importer_gltf.h because travis complains about unitialized variable

./core/cowdata.h:140:3: error: 'accessor.EditorSceneImporterGLTF::GLTFAccessor::sparse_indices_buffer_view' may be used uninitialized in this function [-Werror=maybe-uninitialized]

[akien-mga]

Both @hpvb and @reduz say that it seems fine as a short term fix for 3.2, unless Vector can be further refactored to avoid this issue altogether.

[akien-mga]

Thanks!