xgl: update xglDbg.h for XGL_INSTANCE

Add XGL_INSTANCE as the first parameter of xglDbgRegisterMsgCallback(),
xglDbgUnregisterMsgCallback(), and xglDbgSetGlobalOption().

The loader is simplified.  loader_log() is modified to always output to
stderr.

glave is modified to register its message callback after xglEnumerateGpus(),
and xglDbgUnregisterMsgCallback() is called only before xglDestroyInstance().

XGL_INSTANCE is ignored by the layers and the driver for now.

Bump up API version to XGL_MAKE_VERSION(0, 56, 1).
diff --git a/tests/render_tests.cpp b/tests/render_tests.cpp
index 78052c8..7112f51 100644
--- a/tests/render_tests.cpp
+++ b/tests/render_tests.cpp
@@ -444,7 +444,7 @@
 void XglRenderTest::XGLTriangleTest(const char *vertShaderText, const char *fragShaderText, const bool rotate)
 {
 #ifdef DEBUG_CALLBACK
-    xglDbgRegisterMsgCallback(myDbgFunc, NULL);
+    xglDbgRegisterMsgCallback(inst, myDbgFunc, NULL);
 #endif
     // Create identity matrix
     int i;