bugfix [#22581]
- on instances last hit optimization was jumping to the last hit face
inside an instance without doing space transformation.

Fixed by making last hit optimization jump to the root node of the instance
instead of the last hit face.

Fix [#19602] Rayshadow that goes through transparent plane disappears
(some bugs may show on other places if some of the code isnt prepared to handle non-normalized is->vec)

Fix #19571 (reported by Markus Ilmola):
Added missing callbacks. Was leading to crashs when the raytree was empty.

Fixed #19571
(was crashing when building a raytree from a scene withouth any geometry)

Last fix still failed to link under mingw.
This time it works! (gcc and mingw can handle compile and link)

*Added memset's to make sure counters start on zero
*Disabled ray counter (can be enabled on render/extern/include/RE_raytrace.h by commenting out the define)
*marked bvh_node_merge() as static inline (hopping it now compiles on gcc and mingw)

Merged Soc 2009 - raytrace optimization [0]
from branch [1] at rev 23647

[0] - http://wiki.blender.org/index.php/User:Jaguarandi/SummerOfCode2009/
[1] - https://svn.blender.org/svnroot/bf-blender/branches/soc-2009-jaguarandi

svn merge -r 23528:23646 https://svn.blender.org/svnroot/bf-blender/trunk/blender

Added #ifdef __SSE__ so it can still build when SSE is disabled at compile time

blibvh safe for 64bits