Replaced tomo.Image with tomo.Canvas and tomo.Pattern

This is the first step in transitioning the API over to the new
design. The new tomo.Canvas interface gives drawing functions
direct access to data buffers and eliminates overhead associated
with calling functions for every pixel.

The entire artist package will be remade around this.

elements/testing/mouse.go

@@ -13,7 +13,7 @@ type Mouse struct {
 	*core.Core
 	core core.CoreControl
 	drawing      bool
-	color        tomo.Image
+	color        artist.Pattern
 	lastMousePos image.Point
 }
 
@@ -33,11 +33,11 @@ func (element *Mouse) Handle (event tomo.Event) {
 		element.core.AllocateCanvas (
 			resizeEvent.Width,
 			resizeEvent.Height)
-		artist.Rectangle (
+		artist.FillRectangle (
 			element.core,
 			theme.AccentImage(),
-			artist.NewUniform(color.Black),
-			1, element.Bounds())
+			element.Bounds())
+		// TODO: draw a stroked rectangle around the edges
 		artist.Line (
 			element.core, artist.NewUniform(color.White), 1,
 			image.Pt(1, 1),