add autorescaling to avoid image creation errors

[vincentsarago]

closes #390

This PR does:

• add data type rescaling when we know the output driver will raise an error (e.g float -> uint8 for PNG)

Why the auto-rescaling is only in ImageData.render and not in rio_tiler.utils.render?

The rescale method (rio_tiler.utils.linear_rescale) requires a mask in argument but in rio_tiler.utils.render the mask is either optional or deleted. In the ImageData object, the mask will always be defined.