Dimensions struct bundling width and height

[jofas]

width and height variables throughout the codebase should be replaced with a serializable dimensions struct (best put in the src/util/frame.rs file):

struct Dimensions {
  width: usize,
  height: usize,
}

This should include the Frame struct:

mgart/src/util/frame.rs

Lines 17 to 22 in 0c7ddbb

	#[derive(Debug, Clone)]
	pub struct Frame<T> {
	width: usize,
	height: usize,
	buf: Vec<T>,
	}

And the jsonnet API