[Flutter GPU] Allow specifying the mip level/slice for color attachments.

- Support needs to be added to Impeller's HAL for this.
- Allow specifying a specific `ColorAttachment.mipLevel` (default 0) and `ColorAttachment.slice` (default 0).
- Throw a Dart exception if `!(0 < mipLevel < mipLevelCount)` or if `slice` is is out of range for the given `TextureType` (i.e. `TextureType.cube` has 6 slices, whereas `TextureType.2d` has 1).