fix: Allow selection planes to be picked from both sides

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

src/ViewportWidget.cpp

@@ -798,6 +798,7 @@ void ViewportWidget::drawSelectionPlanes()
     const float planeSize = 5.0f;
     const float planeOffset = 1.0f;
 
+    glDisable(GL_CULL_FACE);
     glEnable(GL_BLEND);
     glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
     glLineWidth(2.0f);
@@ -859,6 +860,7 @@ void ViewportWidget::drawSelectionPlanes()
     glVertex3f(0, planeOffset, planeOffset + planeSize);
     glEnd();
 
+    glEnable(GL_CULL_FACE);
     glEnable(GL_DEPTH_TEST);
     glDisable(GL_BLEND);
 }