Fixed bug 3975 - Add GLES2 support for macOS via ANGLE library

Andrey

Seems latest google angle library successfully built & tested under macOS'es.

https://github.com/google/angle

We need to use GLES2 to implement true cross-platform code.

CMakeLists.txt

@@ -1486,6 +1486,13 @@ elseif(APPLE)
         set(SDL_VIDEO_RENDER_OGL 1)
         set(HAVE_VIDEO_OPENGL TRUE)
       endif()
+
+      if(VIDEO_OPENGLES)
+        set(SDL_VIDEO_OPENGL_EGL 1)
+        set(SDL_VIDEO_OPENGL_ES2 1)
+        set(SDL_VIDEO_RENDER_OGL_ES2 1)
+        set(HAVE_VIDEO_OPENGLES TRUE)
+      endif()
     endif()
   endif()