Fix always switching back to mgun in Q3 controls menu

Caused by 120e296a74b0317f29d7f516d904bdc70f71b006.
Fix Gauntlet barrel axis in UI

UI's PositionRotatedEntityOnTag is different than CGame's and
UI has switched pitch/roll for Gauntlet/BFG axis to get it to
look like /close to/ how it looks in CGame.

Making UI use the same *PositionRotatedEntityOnTag and axis as
CGame fixes the Gauntlet blade being wobbly in controls menu.
mint-arena: Allow any weapon to use client color, not just rail gun

The color is only used if shader uses rgbGen entity. No reason to
limit to railgun. No other Q3 weapon shaders use rgbGen entity,
so doesn't change visuals.
mint-arena: Fix weapon fallback for missing model in UI
mint-arena: Try to load barrel for any weapon in UI

This is what is done in CGame.