internal/mipmap: refactoring
internal/mipmap: refactoring

This change replaces the member `volatile` with `imageType` to make
the code more explicit.

In the old code, it was not obvious whether a mipmap was used for a
screen image. Actually a mipmap was not used since `canSkipMipmap` is
always true for a screen image, but this was too tricky.

hajimehoshi/ebiten,hajimehoshi/ebiten