Issue title: ebiten: add `ReadPixels` to `Image`
 Issue body: This gets pixels and is more efficient than `At`.
 Comments: 
 Comment 0: Or `ReadPixels`?
 Comment 1: > Or ReadPixels?

I like `ReadPixels`!
Still with a `ReadPixels(pixels []byte) []byte` prototype?
 Comment 2: No, this will be `ReadPixels([]byte)`
 Comment 3: I think I'll rename `ReplacePixels` to `WritePixels` for consistency.
{'source': 'GithubIssues', 'repo_name_with_owner': 'hajimehoshi/ebiten', 'issue_id': 1144705006, 'repo_stars': 7291.0}