Added vertex/index buffers, shaders, etc.

Models are rendered with shaders now -> vertex and index buffers
can be (are) used for models (no longer fixed pipeline!).

Material names in .obj files are used to determine textures for
submeshes. Should propably implement separate material that defines
the texture(s) for the submesh.

Improvements and changes made to resource cache, texture cache, and
other files/classes too.

Added textures for the house.obj.
Added ground.obj: A simple terrain mesh.
Added default.shader: Only a test shader.