Refactor image display method for improved performance.

[lbussell]

This pull request does the following:

• Renames the ImageCanvas component to EditorCanvas.
• Creates a new class, ImageCanvas (and another class called PixelGrid).
• Instead of redrawing every pixel every time the window is resized or the image scale changes, we move the image drawing functionality to the new ImageCanvas class, and simply draw the data from ImageCanvas to EditorCanvas all at once without recomputing the colors for every pixel.