Merge pull request #24 from Unity-Technologies/zxw/fix_text_resize_window_issue

fix text rendering issue + update readme

engine/README.md

 Update out/Debug/args.gn with the following content:
 ```
 clang_win = "C:\Program Files\LLVM"
+win_vc = "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC"
 cc = "clang"
 cxx = "clang++"
 is_debug = true

engine/src/shell/platform/unity/unity_surface_manager.cc

 
   image_texture->Release();
 
+  GLint old_texture_binding_2d;
+  glGetIntegerv(GL_TEXTURE_BINDING_2D, &old_texture_binding_2d);
+
   FML_DCHECK(fbo_texture_ == 0);
   glGenTextures(1, &fbo_texture_);
   glBindTexture(GL_TEXTURE_2D, fbo_texture_);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
   glEGLImageTargetTexture2DOES(GL_TEXTURE_2D, fbo_egl_image_);
+  glBindTexture(GL_TEXTURE_2D, old_texture_binding_2d);
+
+  GLint old_framebuffer_binding;
+  glGetIntegerv(GL_FRAMEBUFFER_BINDING, &old_framebuffer_binding);
 
   FML_DCHECK(fbo_ == 0);
   glGenFramebuffers(1, &fbo_);
   FML_CHECK(glCheckFramebufferStatus(GL_FRAMEBUFFER) ==
             GL_FRAMEBUFFER_COMPLETE);
+  glBindFramebuffer(GL_FRAMEBUFFER, old_framebuffer_binding);
 
   return fbo_;
 }