Heavily refactor shell.c.

Ref:
	src/shell.c
		-move components of shell() into individual static functions, for
		greater code modularity.
		-remove g_virtualChanged, in favor of a conditional checking the value
		of g_virtualY (originally -1, which is nonsensical in terms of
		scrolling), to determine whether the use has or hasn't scrolled yet.
		-make Matrix_t *s global variables.