Update imgui dependency.

cmake/LibiglDownloadExternal.cmake

@@ -85,11 +85,11 @@ endfunction()
 function(igl_download_imgui)
 	igl_download_project(imgui
 		GIT_REPOSITORY https://github.com/ocornut/imgui.git
-		GIT_TAG        bc6ac8b2aee0614debd940e45bc9cd0d9b355c86
+		GIT_TAG        v1.66b
 	)
 	igl_download_project(libigl-imgui
 		GIT_REPOSITORY https://github.com/libigl/libigl-imgui.git
-		GIT_TAG        a37e6e59e72fb07bd787dc7e90f72b9e1928dae7
+		GIT_TAG        07ecd3858acc71e70f0f9b2dea20a139bdddf8ae
 	)
 endfunction()