Fix ImporterMesh bone weight handling during lightmap unwrap

[bitsawer]

• Fixes Lightmap UV generation on rigged FBX files causes the mesh to disappear #81636

When unwrapping surfaces for lightmaps we need to update the format flag depending on if we use 0, 4 or 8 influences. Previously this was not done and afterwards the code read invalid memory in some cases (for example seeing invalid bone indices like 545021568 in the bug report).

After this patch, the cube import works. The cube does look pretty busted, but as it is just a test model it might be intentional and bones still seem to affect it correctly when moved. And removing the skeleton influence makes it look like a normal cube. I don't have access to the original model, so @QbieShay might be able to test this more.

Also, I spotted another weird thing with the test project. Simply repeatedly selecting between the Cube mesh node and then another node causes errors and the top UI toolbar keeps growing and adds what looks like a vertical separator every time you do this:

Attempt to disconnect a nonexistent connection from 'test2:<Node3D#1869871115769>'. Signal: 'replacing_by', callable: 'EditorNode::set_edited_scene'.
editor\plugins\node_3d_editor_plugin.cpp:7693 - Condition "p_control->get_parent() != context_toolbar_hbox" is true.
editor\plugins\node_3d_editor_plugin.cpp:7693 - Condition "p_control->get_parent() != context_toolbar_hbox" is true.
editor\plugins\node_3d_editor_plugin.cpp:7693 - Condition "p_control->get_parent() != context_toolbar_hbox" is true.

[bitsawer]

I made this uint64_t instead of uint32_t as future proofing for #81138

[clayjohn]

Looks great! Tested locally and it appears to work.

I get the same janky cube that you get, but when moving bones around it deforms in a way that seems right.

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.1.3.