fix uninitialized variable in btMultiBody

use btAssert to detect 0 determinant in btMatrix3x3 inverse
Remove obsolete comment in PosixSharedMemory, the Windows shared memory implementation was done over a year ago

examples/OpenGLWindow/SimpleOpenGL3App.cpp

@@ -172,7 +172,7 @@ SimpleOpenGL3App::SimpleOpenGL3App(	const char* title, int width,int height, boo
     
     b3Assert(glGetError() ==GL_NO_ERROR);
 
-	m_instancingRenderer = new GLInstancingRenderer(128*1024,64*1024*1024);
+	m_instancingRenderer = new GLInstancingRenderer(128*1024,128*1024*1024);
     m_primRenderer = new GLPrimitiveRenderer(width,height);
     
     m_renderer = m_instancingRenderer ;