refactor: refactor mesh generation for IconMeshFactory

[pollend]

This omits the use of the TessellatorHelper, and just defines the mesh from StandardMeshData in a private method call. This should be more straightforward to follow. We end up losing some of the flexible code from TessellatorHelper but I think this logic is a lot more easier to follow albeit repetitious.

This can be verified by running core gameplay and verifying that the mesh for an icon item is generated properly as it appears in the inventory and in the player hand.

[jdrueckert]

The icons are generated properly in the inventory, but I noticed that when in-hand, the items are not rendered with shadows anymore, which IMO looks not as good as before...
develop:

this branch:

[jdrueckert]

Looks better now 👍 It's still a bit brighter than it was before but I think that's actually a good thing, because now you can identify more details also in darker items.