Dependency between igl::png and igl::opengl

Former-commit-id: ae65c94c52ee70c24e3487913c0b0e081777cebf

shared/cmake/libigl.cmake

@@ -360,12 +360,16 @@ endif()
 ################################################################################
 ### Compile the png parts ###
 if(LIBIGL_WITH_PNG)
-  set(STB_IMAGE_DIR "${LIBIGL_EXTERNAL}/stb_image")
-  if(NOT TARGET stb_image)
-    add_subdirectory("${STB_IMAGE_DIR}" "stb_image")
+  if(TARGET igl_opengl)
+    set(STB_IMAGE_DIR "${LIBIGL_EXTERNAL}/stb_image")
+    if(NOT TARGET stb_image)
+      add_subdirectory("${STB_IMAGE_DIR}" "stb_image")
+    endif()
+    compile_igl_module("png" "")
+    target_link_libraries(igl_png ${IGL_SCOPE} igl_stb_image igl_opengl)
+  else()
+    set(LIBIGL_WITH_PNG OFF CACHE BOOL "" FORCE)
   endif()
-  compile_igl_module("png" "")
-  target_link_libraries(igl_png ${IGL_SCOPE} igl_stb_image)
 endif()
 
 ################################################################################