[Impeller] OpenGLES MSAA Follow-up: Avoid using `GL_DEPTH_STENCIL`

```cc
// texture_gles.cc

// TODO(matanlurey): This is a combined depth stencil format (like
// kD24UnormS8Uint below). We should find a way to use a stencil-only
// format instead.
case PixelFormat::kS8UInt:
  internal_format = GL_DEPTH_STENCIL;
  external_format = GL_DEPTH_STENCIL;
  type = GL_UNSIGNED_INT_24_8;
  break;
```