A3DGE Code

* Step back on the Billboard/Sprite situation: Now there's only TBillboard
with a property that changes how it's rendered (as billoboard or as sprite).

* Differentiated "billboard" from "Sprite 3D", adding a planar model to simplify.
* Documentation fixes and updates.
* Octree "Childs" renamed to "Children".

+ Added unit a3dge.Collisions that implements routines used to check collisions
between objects.

* TOctree.CheckCollisions checks object radius and ignores if it's zero or less.
+ Added a few explanations in the examples headers and a few more aestetic
changes..
+ Added example ex_collision.

* Property OnCollideWith renamed to OnCollision.
* Fixed method TOctreeNode.Inside when using bounding box.
* TOctree.CheckCollisions checks if objects have OnCollision assigned now.
* TObject3D.SetBoundingBox sets Radius if not set.
* Fixed TObject3D.CheckCollision.
* TObject3D.RenderBoundingVolume now renders only one of the volumes (sphere or
box) depending the value of UseBoundingBox.
* Documentation updates, mostly collision related.

* Fixed order of box limits on octree, model and object methods to follow the
gnomon (-x, +x, -y, +y, -z, +z). Old order was confusing and I've found a few
mistakes in octree code; I suspect there are more in collision code.
- Removed a compilation warning in a3dge.heightmap.
+ Added a new option to makefile.
+ Some additions in examples ex_world and ex_test.

+ Added functions AreSameVector3D and AreDifferentVector3D to a3dge.Math3D.
+ Added method that calculates bounding boxes for polyhedrons.
* Fixed CreateBox (Duh!).
- Removed method TObject3D.CollidedWith.
+ Added event TObject3D.onCollideWith.
* Fixed bounding volume rendering (Duh!).
* Some changes to the collision checking code to deal with onCollideWith, but
seems it doesn't work.
* A few changes to ex_world to make it cleaner.

+ Added example of use of TWorld3D.

* Fixed transparent object rendering.
+ Added method "ScreenShot" to TDisplay.

* Fixed GetVectorLength2 (Duh!).
+ Added property "Transparent" to models.
* World3D tries to render transparent and translucent objects properly but it
fails (for example: set a3dge.world3d.MaxTransparentObjects = 255 and
ex_text.NumObjects = 256).