Reduce stacking potential of melee damage multipliers

[Mirsario]

Thorium's Sword Sheaths, while very overpowered even on their own, become much more ridiculous with Overhaul on:

https://github.com/Mirsario/TerrariaOverhaul/assets/6795251/3eac10e7-ca24-4973-8514-bd0ff531d2ed

Thorium or not, it's fitting to alter Overhaul's multipliers to stack in an additive rather than multiplicative way, making two x1.5 multipliers stack as x2.0 rather than x2.25.