fix: disambiguate Drop Target button IDs in Drag'N'Drop demo (#387)

The demo renders three drop-target buttons whose labels share the same
value of the mutable 'data' field, so imgui hashes three identical
item IDs within the same window. This was silently broken pre-1.91
(drop zones shared hover/active state) and surfaces as a visible
"3 visible items with conflicting ID!" warning now that imgui 1.91's
ConfigDebugHighlightIdConflicts is on by default in debug builds.

Wrap each drop-target block in a PushID("any"/"string"/"class") /
PopID() pair so the buttons share a human-readable label but are
distinguished in the ID stack. This is the resolution imgui's own
warning recommends first.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: phraktle

example/src/main/java/ExampleDragAndDrop.java

@@ -34,20 +34,26 @@ public static void show(final ImBoolean showDragNDropWindow) {
             ImGui.separator();
 
             ImGui.text("Drop here any:");
+            ImGui.pushID("any");
             ImGui.button(data, 100, 50);
             setupTarget();
+            ImGui.popID();
 
             ImGui.separator();
 
             ImGui.text("Drop here string payload only");
+            ImGui.pushID("string");
             ImGui.button(data, 100, 50);
             setupStringPayloadTarget();
+            ImGui.popID();
 
             ImGui.separator();
 
             ImGui.text("Drop here class specific payload only");
+            ImGui.pushID("class");
             ImGui.button(data, 100, 50);
             setupClassSpecificPayloadTarget();
+            ImGui.popID();
         }
         ImGui.end();
     }