display: disable depth test before DisplayRegion draw callback

Having depth test disabled is the default OpenGL state, and callbacks may quite reasonably expect to see the default state.  Kivy seems to expect this, for one.

panda/src/display/graphicsEngine.cxx

@@ -47,6 +47,7 @@
 #include "displayRegionCullCallbackData.h"
 #include "displayRegionDrawCallbackData.h"
 #include "callbackGraphicsWindow.h"
+#include "depthTestAttrib.h"
 
 #if defined(WIN32)
   #define WINDOWS_LEAN_AND_MEAN
@@ -2046,9 +2047,12 @@ do_draw(GraphicsOutput *win, GraphicsStateGuardian *gsg, DisplayRegion *dr, Thre
   if (cbobj != nullptr) {
     // Issue the draw callback on this DisplayRegion.
 
-    // Set the GSG to the initial state.
+    // Set the GSG to the initial state.  We disable depth testing since that
+    // is the default OpenGL state, and some libraries (eg. Kivy) expect that.
+    static CPT(RenderState) state = RenderState::make(
+      DepthTestAttrib::make(DepthTestAttrib::M_none));
     gsg->clear_before_callback();
-    gsg->set_state_and_transform(RenderState::make_empty(), TransformState::make_identity());
+    gsg->set_state_and_transform(state, TransformState::make_identity());
 
     DisplayRegionDrawCallbackData cbdata(cull_result, scene_setup);
     cbobj->do_callback(&cbdata);