PBR for models

[JKSunny]

Currently only world surfaces allow for pbr shaders/materials.

Because tangents are not yet extracted and uploaded for models.
This also requires to set normals and tangents vertex stride properly.

• MD3 support (might need to increase com_hunkmegs > 256, tangents and normals use vec4_t not uint16_t)
• IQM support (added tangents, untested)
• MDR support

optionally:
would be nice if #2 is implemented additionally.