fix(lwjgl3): ensure GL device objects before renderDrawData

Since 1.87, ImGuiImplGl3 creates the shader/program/VBO and font texture
lazily in newFrame() rather than in init(), matching upstream. Call sites
that only call init() + renderDrawData() (the pre-1.87 pattern used by
many jME / custom loops) then hit a native NULL dereference inside
glDrawElementsBaseVertex.

Share the lazy create path with renderDrawData so those upgrades no
longer crash. Prefer calling newFrame() every frame; this is a safety
net for the upgrade path reported in #361.

Fixes #361

Author: arimu1

imgui-lwjgl3/src/main/java/imgui/gl3/ImGuiImplGl3.java

@@ -242,6 +242,10 @@ public boolean init() {
      * Method to do an initialization of the {@link ImGuiImplGl3} state.
      * It SHOULD be called before calling of the {@link ImGuiImplGl3#renderDrawData(ImDrawData)} method.
      * <p>
+     * GL device objects (shader, buffers, font texture) are <b>not</b> created here; they are
+     * created lazily on the first {@link #newFrame()} or {@link #renderDrawData(ImDrawData)} call
+     * (behavior since 1.87, matching upstream). Prefer calling {@link #newFrame()} every frame.
+     * <p>
      * Method takes an argument, which should be a valid GLSL string with the version to use.
      * <pre>
      * ----------------------------------------
@@ -371,7 +375,25 @@ public void shutdown() {
         data = null;
     }
 
+    /**
+     * Prepare per-frame GL resources.
+     * <p>
+     * Since Dear ImGui 1.87 / imgui-java 1.87, shader program, buffers and the font
+     * texture are created lazily here (not in {@link #init(String)}). Call this every
+     * frame before {@link ImGui#newFrame()}, or at least once after {@code init} and
+     * before the first {@link #renderDrawData(ImDrawData)}.
+     */
     public void newFrame() {
+        ensureDeviceObjects();
+    }
+
+    /**
+     * Create shader/program/VBO and font texture if they are not yet available.
+     * Shared by {@link #newFrame()} and {@link #renderDrawData(ImDrawData)} so that
+     * integrators that only call {@code init} + {@code renderDrawData} (the pre-1.87
+     * pattern) do not hit a native NULL dereference in {@code glDrawElementsBaseVertex}.
+     */
+    protected void ensureDeviceObjects() {
         if (data.shaderHandle == 0) {
             createDeviceObjects();
         }
@@ -470,6 +492,12 @@ public void renderDrawData(final ImDrawData drawData) {
             return;
         }
 
+        // Device objects used to be created in init() (imgui-java <= 1.86). Since 1.87 they are
+        // created lazily in newFrame() to match upstream imgui_impl_opengl3. Ensure them here as
+        // well so call sites that skip newFrame() (common after upgrades; see #361) do not crash
+        // with a native NULL pointer inside glDrawElementsBaseVertex.
+        ensureDeviceObjects();
+
         // In C++: iterates draw_data->Textures and calls ImGui_ImplOpenGL3_UpdateTexture for each non-OK status.
         // In Java: ImTextureData is not exposed in imgui-binding (follow-up); we keep the legacy createFontsTexture
         //          path triggered from newFrame(), so dynamic atlas updates are not honored here yet.