Fixed bug in MTLLoader.js

Before this fix, there was this error in the console:
> Object [object Object] has no method 'loadTexture'

for these examples files:
* examples/webgl_loader_scene.html
* examples/webgl_loader_scene.html
* examples/webgl_loader_utf8.html

```loadTexture()``` was attached to the wrong prototype.

After this fix, all the above example files load without errors. The visual results look correct in the viewport too.