Add updateBoundingBox parameter to IMeshBuffer::append and some fixes and optimizations

Fix: When appending to an empty mesh boundingbox has to be initialized with first position
Adding updateBoundingBox parameter as there is a bit costs involved in updating that and it might not be necessary at that point
Default is still to do it - and with the default parameter it's at least compile compatible to old interface (unless users created their own meshbuffers).
Optimizing the copying of vertices in CDynamicMeshBuffer::append by using memset when possible instead of pushing each vertex (which goes through quite a few virtual functions)

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@6496 dfc29bdd-3216-0410-991c-e03cc46cb475

changes.txt

@@ -1,6 +1,7 @@
 --------------------------
 Changes in 1.9 (not yet released)
 
+- Get IMeshBuffer::append functions working for a few more cases and adapt interface so one can prevent the BoundingBox update.
 - Bugfix: SMaterialLayer::operator!= no longer returns true when comparing a layer without texture matrix with one with a set identity texture matrix. Those are the same.
 - SMaterialLayer no longer releases allocated texture memory before destructor unless explicitly requested.
   This can avoid constantly allocation/releasing memory in the active driver material when setting materials.