Ensure GL device objects before renderDrawData after 1.86→1.87 upgrade

[arimu1]

Summary

Since imgui-java 1.87, ImGuiImplGl3 creates the shader/program/VBO and font texture lazily in newFrame() (not in init()), matching upstream imgui_impl_opengl3. Integrators that still use the pre-1.87 pattern of init() + renderDrawData() without newFrame() then crash in native code (glDrawElementsBaseVertex NULL dereference / EXCEPTION_ACCESS_VIOLATION).

This change reuses that lazy create path from renderDrawData() as a safety net, so the upgrade path reported in #361 no longer hard-crashes the JVM. Calling newFrame() every frame remains the recommended API.

Type of change

• Minor changes or tweaks (quality of life stuff)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Notes for reviewer

• Root cause of Bug: An "null pointer" error occurred. #361: device objects never created when newFrame() is skipped; shaderHandle / buffer handles stay 0 and the draw call faults in the GL driver.
• The issue author later worked around it by calling imGuiGl3.newFrame() during init (see their jME hub write-up and @Enaium's comment). This patch keeps that recommendation and additionally makes renderDrawData() self-sufficient for the upgrade path.
• No new public API beyond a small protected ensureDeviceObjects() helper shared by newFrame() / renderDrawData().
• Javadoc on init / newFrame updated to document the post-1.87 lazy create.

Fixes #361

Test plan

• ./gradlew :imgui-lwjgl3:compileJava :imgui-lwjgl3:checkstyleMain (JDK 17)
• No automated GL unit test exists for this path; visual smoke would be ./gradlew :example:run with local natives if desired