Model support for collision meshes

[walbourn]

The existing CMO and SDKMESH model loaders create IBs and VBs as USAGE_DEFAULT and then release the loaded geometry. This is a great match for rendering, but makes it difficult to do CPU-side triangle-based collision.

There should be a way to get Model to keep a shadow-copy or a parameter to inform the system that the loaded model is for collision and not intended for rendering...

[walbourn]

D3DX's Mesh Intersect performed the following test.

• Perform a ray-hit test against the bounding box of the mesh. If it missed, no hits. If it hit, further processing.
• Allocate an array of Booleans with the same number of entries as the maximum vertex index in the mesh.
• Generates a plane containing the test ray, and store if the vertex was 'in front' or 'behind' that test plane.
• For each triangle in the mesh, first check to see if all vertices in the triangle are on the same 'side' of the test plane by looking in that array. If they are, skip testing that triangle because it can't hit the ray.
• Compute triangle hit test. All hits are stored in an array in sorted order.

[walbourn]

Could support by adding a 'flags' option to Model::CreateFrom* to include collision data in the form of two simple std::vector arrays in ModelMesh.