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

include/SSharedMeshBuffer.h

@@ -187,13 +187,13 @@ namespace scene
 
 
 		//! append the vertices and indices to the current buffer
-		virtual void append(const void* const vertices, u32 numVertices, const u16* const indices, u32 numIndices)  IRR_OVERRIDE 
+		virtual void append(const void* const vertices, u32 numVertices, const u16* const indices, u32 numIndices, bool updateBoundingBox)  IRR_OVERRIDE 
 		{
 			// can't do that as it doesn't own the vertex memory
 		}
 
 		//! append the meshbuffer to the current buffer
-		virtual void append(const IMeshBuffer* const other) IRR_OVERRIDE 
+		virtual void append(const IMeshBuffer* const other, bool updateBoundingBox) IRR_OVERRIDE 
 		{
 			// can't do that as it doesn't own the vertex memory
 		}