Support RTCW style combined legs/torso with IQM models

[zturtleman]

Return to Castle Wolfenstein and Enemy Territory player models use refEntity_t torsoAxis, torsoFrame and R_LerpTag() / trap_R_LerpTagTorso() to specify torso rotation and animation frame in a combined legs/torso "body" model. This has the benefit of a seamless waist compared to Quake 3's separate rotating legs and torso models.

Currently this is only supported for RTCW's MDS and ET's MDX/MDM formats. It should be possible to support for IQM models. Maybe by rotating the parent joint of tag_torso joint and using torso animation for its child joints (except tag_torso?). I need to look at MDS behavior.