Fix bad cast in CIndexBuffer. Modify IIndexBuffer interface for cleanup and safety.

CIndexBuffer::setType could end up casting a temporary u16 variable to u32& (reference!).
Probably never noticed as this tended to mostly works (guess next byte on stack often 0?).
To prevent this from happening again I modifed IIndexBuffer::push_back to work with copies instead of references.
While breaking the interface anyway I also deprecated pointer() which is just identical to getData() anyway.
I get the idea of staying similar to core::array interface, but it's just confusing (and not same due to lack of types anyway).
Also added a const version for getData()

On a side-note - same setType bug still in CVertexBuffer, but a bit harder to fix there. So will be an new patch.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@6357 dfc29bdd-3216-0410-991c-e03cc46cb475

changes.txt

@@ -1,6 +1,9 @@
 --------------------------
 Changes in 1.9 (not yet released)
 
+- IIndexBuffer has some interface changes: pointer() deprecated (was same as getData()). Adding a const version of getData(). 
+  Also some functions no longer use references but copy by value which shouldn't be slower for this and is safer to use.
+- Fix CIndexBuffer::setType going from 16 to 32 bit. This had some casts which simply could mess up the results.
 - Fix OSX nor resizing properly. Thanks @torleif, Jordach and sfan5 for patch and report: https://irrlicht.sourceforge.io/forum/viewtopic.php?f=2&t=52819
 - X meshloader fixes bug with uninitialized normals. Thanks @sfan5 for patch: https://irrlicht.sourceforge.io/forum/viewtopic.php?f=2&t=52819
 - stl meshloader now faster, especially with text format