Improve text sharpness in DrawingIsland sample

[niklasb-ms]

Description

This improves text sharpness of text in the DrawingIsland sample by ensuring the surface brush is not scaled or subpixel-positioned. The specific changes are:

• Set the drawing surface width and height to be a multiple of 1 pixel.
• Set the visual width and height to exactly match (but in DIPs rather than pixels).
• Enable pixel snapping for the visual.