[Undo]
Implemented for "Point Brush"
Merge branch 'master' of https://github.com/ywangbc/COMP4411

Conflicts:
	impressionist.sdf
[Undo] | [Greyscale Brush] | [Contrast Brush]
Undo:
implemented undo once for all brushes
(TODO - implement unlimited undo using stack)

GreyscaleBrush, ContrastBrush:
replaced point brush by scattered point brush
(for better illustration)
[Undo]
Implemented unlimited undo (Ctrl + z)