Add mdl loader

http://tfc.duke.free.fr/coding/mdl-specs-en.html

```
char: 1 byte
short: 2 bytes
int: 4 bytes
float: 4 bytes
ubyte: 1 unsigned byte
```

```
/* MDL header */
struct mdl_header_t
{
int ident; /* magic number: "IDPO" */
int version; /* version: 6 */

vec3_t scale; /* scale factor */
vec3_t translate; /* translation vector */
float boundingradius;
vec3_t eyeposition; /* eyes' position */

int num_skins; /* number of textures */
int skinwidth; /* texture width */
int skinheight; /* texture height */

int num_verts; /* number of vertices */
int num_tris; /* number of triangles */
int num_frames; /* number of frames */

int synctype; /* 0 = synchron, 1 = random */
int flags; /* state flag */
float size;
};
```