Fix drawing of 2D skeletons in the RD renderer.

[clayjohn]

Fixes: #59451
Fixes: #35726

There were multiple issues:

1. The renderer expects bones if a mesh uses a Skeleton. So if we don't have any bones assigned yet, do not register the skeleton with the RenderingServer (thats the change in polygon_2d.cpp)
2. The skeleton shader wasn't writing any values out at the end (simply add a write to the destination vertex buffer)
3. The skeleton shader erroneously had some values copied from the 3D version (change stride from 3 to 2)
4. Skeletons were culled if the rest pose went outside of the viewport

To solve the culling issue we do 2 things:

1. When a canvas item has a skeleton we always request the AABB from the rendering server
2. In the rendering server we check if the skeleton has changed since the last time the AABB was generated. If it has changed, we regenerate the AABB. This fix is similar to Fix skeleton 2D stale bounding rect #63071 except in 4.0 we can keep the logic close to where the mesh and skeleton are updated

The first two problems stopped Skeletons from working at all. After I solved those I discovered the other problems and slowly added to this PR.

The rest of the code is removing dead code that hasn't been doing anything. I'm doing this in preparation for fixing up some other lingering problems with 2D skeletons and adding skeletons to GLES3. Now that Skeletons are working well in the RD renderer I can add Skeletons to the GLES3 renderer

[akien-mga]

Thanks!

[akien-mga]

Fixes: #35726

Is that backportable for 3.x?