ebiten: enable to pass a regular `image.Image` to `DrawImage` and so on

[hajimehoshi]

This is still a draft.

• Ebitengine can have an internal *ebiten.Image cache for image.Image
• If there is the same size *ebiten.Image as the given image.Image, pick it, call ReplacePixels, and then use it as a rendering source
  • Unfortunately ReplacePixels should be called every time even if the image is not modified.
• If there is not, create a new *ebiten.Image
• Check the lifetime of the cached *ebiten.Image objects. If one is too old, dispose it and remove it from the cache.

Probably the concern is that this could cause a performance issue that is hard to investigate...

[hajimehoshi]

/CC @eihigh

[hajimehoshi]

If the composition mode is 'Copy', DrawImage would work as 'ReplacePixels'.

[tinne26]

While I can see there are some benefits to setting up a cache, I think the alternative implementation without caching should also be considered. I think simply documenting that using images that are not *ebiten.Image can fall back to slow paths is acceptable. Users can still create their own *ebiten.Images when necessary. This is the same that happens today, albeit with multiple functions (DrawImage / Set / ReplacePixels) instead of just one.

[hajimehoshi]

The concern is that users might abuse DrawImage with image.Image even though we warn such usages are slow. Actually Set was used more often than I expected... Set didn't have such warnings so this was my responsibility, but I think basically we cannot control how often they are used as long as the API is easy to access.

[hajimehoshi]

This would contribute to the API's easiness, but make its simplicity worse. This is opposite to my policy unforutnately.