Fix crash on startup due to uninitialized background texture ID

- Manually assigned OpenGL texture ID to backgroundImage in initTextures().
- Registered the texture with OpenGL via setTextureLookup(), as done for other manual textures.
- Resolves assertion failure in getId() when background image was enabled.

Author: Sunnyl75

src/display/Textures.cpp

@@ -257,6 +257,13 @@ void MapCanvas::initTextures()
 
     textures.backgroundImage = loadTexture(getPixmapFilenameRaw("background-image.png"));
 
+    if (textures.backgroundImage) {
+        auto &tex = deref(textures.backgroundImage);
+        const auto id = allocateTextureId();
+        tex.setId(id);
+        m_opengl.setTextureLookup(id, textures.backgroundImage);
+    }
+
 
     {
         textures.for_each([this](SharedMMTexture &pTex) -> void {