internal/restorable: remove `ImageTypeVolatile`

[hajimehoshi]

Operating System

• [ ] Windows
• [ ] macOS
• [ ] Linux
• [ ] FreeBSD
• [ ] OpenBSD
• [X] Android
• [ ] iOS
• [ ] Nintendo Switch
• [ ] PlayStation 5
• [ ] Xbox
• [ ] Web Browsers

What feature would you like to be added?

Now we have Hint which is for optimization when overwriting some regions on a target texture, an image which is cleared every frame should no longer be stale. Thus, we don't need a notion of being volatile. In order to simplify the code, let's try to remove this.

Why is this needed?

To simplify the code.

[hajimehoshi]

an image which is cleared every frame should no longer be stale

Unfortunately this is not true when too many draw calls happen on the same image.