Motivation

• I want decode_rect and Decoder::read_surface_rect to take an ImageViewMut to make the APIs simpler.
• This also allows adding a crop API
• This makes encode and decode more general.

Description

Add a row pitch field/parameter for the both ImageView and ImageViewMut.

The main problem that comes with this is that most de/encoder implementation needs to be updated. They currently all assume that row_pitch = width * bytes_per_pixel. This is especially true for compressed formats, which have optimized paths where we can directly pass the image buffer to the reader/writer.