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/basic/container.go

@@ -57,7 +57,7 @@ func (element *Container) Adopt (child tomo.Element, expand bool) {
 			
 			return
 		},
-		Draw: func (region tomo.Image) {
+		Draw: func (region tomo.Canvas) {
 			element.drawChildRegion(child, region)
 		},
 	})
@@ -318,10 +318,9 @@ func (element *Container) recalculate () {
 func (element *Container) draw () {
 	bounds := element.core.Bounds()
 
-	artist.Rectangle (
+	artist.FillRectangle (
 		element.core,
 		theme.BackgroundImage(),
-		nil, 0,
 		bounds)
 
 	for _, entry := range element.children {
@@ -329,7 +328,7 @@ func (element *Container) draw () {
 	}
 }
 
-func (element *Container) drawChildRegion (child tomo.Element, region tomo.Image) {
+func (element *Container) drawChildRegion (child tomo.Element, region tomo.Canvas) {
 	if element.warping { return }
 	for _, entry := range element.children {
 		if entry.Element == child {

elements/basic/label.go

@@ -93,10 +93,9 @@ func (element *Label) updateMinimumSize () {
 func (element *Label) draw () {
 	bounds := element.core.Bounds()
 
-	artist.Rectangle (
+	artist.FillRectangle (
 		element.core,
 		theme.BackgroundImage(),
-		nil, 0,
 		bounds)
 
 	textBounds := element.drawer.LayoutBounds()