Inefficient algorithm for DrawRect

[lukewilson2002]

The function DrawRect in [ui/drawfunctions.go] makes too many calls to (tcell.Screen).SetContent(). A better algorithm would make a call to SetContent only once per row, and would therefore make use of the (currently set nil) combc parameter of the function. A performance graph has been included below showing the current state of the performance compared to other parts of the editor.

Generated by running the editor with -cpuprofile cpu.prof and to view it: go tool pprof cpu.prof -> web