Buildbot: use relative path for redistributable dlls on windows.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57473 954f8c5b-7b00-dc11-b283-0030488c597c
Fix #35768: crash in with "free image textures" option and 3D viewport with textured
draw mode open. OpenGL texture free needs to happen in the main thread, but it was
freeing a copy of the image datablock. I can't understand how this code ever worked,
probably it never did.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57474 954f8c5b-7b00-dc11-b283-0030488c597c