GUI terminal should not render partial lines.

[andrei-toterman]

As of now, if the size of the GUI terminal is not divisible by the size of a terminal line, the terminal will still render the first/last line but only partially. It should only render the lines that can entirely fit and fill the edges with blank space until a new line can fit. Also, when scrolling, the lines should move in increments of a line height, not smoothly.