Add a way for applications to request canvases

Makes fix to #14 useful

backend.go

@@ -29,6 +29,10 @@ type Backend interface {
 	// reject any texture that was not made by it.
 	NewTexture (image.Image) canvas.TextureCloser
 
+	// NewCanvas creates a new canvas with the specified bounds. The backend
+	// must reject any canvas that was not made by it.
+	NewCanvas (image.Rectangle) canvas.CanvasCloser
+
 	// Run runs the event loop until Stop() is called, or the backend
 	// experiences a fatal error.
 	Run () error

canvas/canvas.go

@@ -77,6 +77,13 @@ type Canvas interface {
 	SubCanvas (image.Rectangle) Canvas
 }
 
+// CanvasCloser is a canvas that can be closed. Anything that receives a
+// CanvasCloser must close it after use.
+type CanvasCloser interface {
+	CanvasCloser
+	io.Closer
+}
+
 // Drawer can draw to a canvas.
 type Drawer interface {
 	// Draw draws to the given canvas.

tomo.go

@@ -93,3 +93,10 @@ func NewTexture (source image.Image) canvas.TextureCloser {
 	assertBackend()
 	return backend.NewTexture(source)
 }
+
+// NewCanvas creates a new canvas with the specified bounds. When no longer in
+// use, it must be freed using Close().
+func NewCanvas (bounds image.Rectangle) canvas.CanvasCloser {
+	assertBackend()
+	return backend.NewTexture(bounds)
+}