Increase height of weapon item bbox from 32 to 48

This prevents players missing weapons by jumping straight over them, which is more consistent with all the other item types, and less frustrating.

Current default physics has a jump height of 42.

This also improves compatibility with CPMA and defrag CPM. A good map to test this on is gu3-pewter. In CPM, you can pick up the weapons in the glass boxes by standing on top. In Xonotic you can't with a weapon height of 32, but you can with 48.

The jump height in Q3/defrag/cpma is 48 and you can't jump straight over items without getting them in CPM.
Edit2: it is possible to jump straight over items in VQ3 and defrag VQ3, but not in CPMA or defrag CPM.

Edit: it also makes trace culling more reliable:

32	48