Fix rendering IQM models between model frames

For lerped frames (refEntity_t frame not equal oldframe) IQM joint
matrices may have incorrect axis scale. This can cause significant model
distortion. The matrix lerp is linear causing each vector to move in a
straight line between frames instead of arcing like a circle. Each joint
frame can have a different scale so can't just normalize the joint
matrix.

Store joints as quaternions and spherical lerp between them and then
convert to a matrix. For my test model, setting up the skeleton is four
times slower now but it still seems to be fast enough to be usable.

code/renderergl2/tr_local.h

@@ -954,6 +954,12 @@ typedef struct srfBspSurface_s
 	float			*heightLodError;
 } srfBspSurface_t;
 
+typedef struct {
+	vec3_t translate;
+	quat_t rotate;
+	vec3_t scale;
+} iqmTransform_t;
+
 // inter-quake-model
 typedef struct {
 	int		num_vertexes;
@@ -988,8 +994,9 @@ typedef struct {
 
 	char		*jointNames;
 	int		*jointParents;
-	float		*jointMats;
-	float		*poseMats;
+	float		*bindJoints; // [num_joints * 12]
+	float		*invBindJoints; // [num_joints * 12]
+	iqmTransform_t	*poses; // [num_frames * num_poses]
 	float		*bounds;
 
 	int		numVaoSurfaces;