... and use them to improve the horizontal_scrolling example.

Now ScrollableBlock can combine a horizontally scrolling viewport with a
horizontal block.

Also rename ViewportScrollDirection to just ScrollDirection for less verbosity.

We were subtracting the scrollOffset when we were supposed to add it.

RenderConstrainedBox was ignoring its additional constraints for its intrinsic size logic when it had no child.

This change makes it easier to defined only the width or the height of an image
and let the other value be filled in from the image's intrinsic aspect ratio.

Fixes #175

This function took innerConstraints as an argument but used constraints internally.