Add a scrollview widget #5

Something to feed numbers into to calculate scroll bar size, scroll thumb size, and scroll thumb position. Returns the part of the scroll view you can render inside.

const scroll_view_body = vxim.scrollView(.{.view_width, .view_height, .x, .y, .content_width, .content_height, .scroll_position });

Can probably be based on rows and column numbers for widths, heights, and scroll position.