Add explicit refentity flag for view weapon?

[zturtleman]

View weapon has it's own field of view for projection (as specific in refdef_t) however view weapon entity is detected by RF_DEPTHHACK (without RF_CROSSHAIR, as 3D crosshair uses both flags). This would interfere with other refentities using RF_DEPTHHACK. For backward compatibility could make existing RF_DEPTHHACK (legacy) value use RF_VIEWWEAPON and new RF_DEPTHHACK.