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.

tomo.go

@@ -2,25 +2,6 @@ package tomo
 
 import "image"
 import "errors"
-import "image/draw"
-import "image/color"
-
-// Image represents a simple image buffer that fulfills the image.Image
-// interface while also having methods that do away with the use of the
-// color.Color interface to facilitate more efficient drawing. This interface
-// can be easily satisfied using an image.RGBA struct.
-type Image interface {
-	image.Image
-	RGBAAt (x, y int) (c color.RGBA)
-}
-
-// Canvas is like Image but also requires Set and SetRGBA methods. This
-// interface can be easily satisfied using an image.RGBA struct.
-type Canvas interface {
-	draw.Image
-	RGBAAt (x, y int) (c color.RGBA)
-	SetRGBA (x, y int, c color.RGBA)
-}
 
 // ParentHooks is a struct that contains callbacks that let child elements send
 // information to their parent element without the child element knowing
@@ -29,7 +10,7 @@ type Canvas interface {
 type ParentHooks struct {
 	// Draw is called when a part of the child element's surface is updated.
 	// The updated region will be passed to the callback as a sub-image.
-	Draw func (region Image)
+	Draw func (region Canvas)
 
 	// MinimumSizeChange is called when the child element's minimum width
 	// and/or height changes. When this function is called, the element will
@@ -49,7 +30,7 @@ type ParentHooks struct {
 }
 
 // RunDraw runs the Draw hook if it is not nil. If it is nil, it does nothing.
-func (hooks ParentHooks) RunDraw (region Image) {
+func (hooks ParentHooks) RunDraw (region Canvas) {
 	if hooks.Draw != nil {
 		hooks.Draw(region)
 	}
@@ -82,10 +63,10 @@ func (hooks ParentHooks) RunSelectabilityChange (selectable bool) {
 
 // Element represents a basic on-screen object.
 type Element interface {
-	// Element must implement the Image interface. Elements should start out
-	// with a completely blank image buffer, and only set its size and draw
+	// Element must implement the Canvas interface. Elements should start
+	// out with a completely blank buffer, and only allocate memory and draw
 	// on it for the first time when sent an EventResize event.
-	Image
+	Canvas
 
 	// Handle handles an event, propagating it to children if necessary.
 	Handle (event Event)