Fix GTK negative content width warnings for bitmap toggle buttons

On Linux/GTK, CheckBox, RadioBox, and SwitchButton set their size to
exactly the bitmap size (18x18 or 16x16), but GTK's internal CSS padding
requires additional space, resulting in negative content width warnings.
Use GetBestSize() on GTK to account for theme padding.

src/slic3r/GUI/Widgets/SwitchButton.cpp

@@ -180,8 +180,15 @@ void SwitchButton::Rescale()
 			(i == 0 ? m_off : m_on).bmp() = bmp;
 		}
 	}
-	SetSize(m_on.GetBmpSize());
 	update();
+#ifdef __WXGTK__
+	wxSize bestSize = GetBestSize();
+	bestSize.IncTo(m_on.GetBmpSize());
+	SetSize(bestSize);
+	SetMinSize(bestSize);
+#else
+	SetSize(m_on.GetBmpSize());
+#endif
 }
 
 void SwitchButton::update()