X backend better handles expose events

Previously, when an expose event was recieved, the backend would
call Window.paste, converting RGBA image data to BGRA image data.
Now we only call Window.pushRegion with the bounds given to us by
the expose event(s). This speeds up window resizing significantly.

artist/patterns/texture.go

@@ -24,22 +24,27 @@ func (pattern Texture) Draw (destination canvas.Canvas, clip image.Rectangle) {
 	srcPoint := bounds.Min.Sub(realBounds.Min).Add(srcBounds.Min)
 	srcPoint.X = wrap(srcPoint.X, srcBounds.Min.X, srcBounds.Max.X)
 	srcPoint.Y = wrap(srcPoint.Y, srcBounds.Min.Y, srcBounds.Max.Y)
-	srcPointXStart := srcPoint.X
 	
 	for dstPoint.Y = bounds.Min.Y; dstPoint.Y < bounds.Max.Y; dstPoint.Y ++ {
-		srcPoint.X = srcPointXStart
+		srcPoint.X = srcBounds.Min.X
+		dstPoint.X = bounds.Min.X
+		dstYComponent := dstPoint.Y * dstStride
+		srcYComponent := srcPoint.Y * srcStride
 		
-		for dstPoint.X = bounds.Min.X; dstPoint.X < bounds.Max.X; dstPoint.X ++ {
-			dstIndex := dstPoint.X + dstPoint.Y * dstStride
-			srcIndex :=
-				srcPoint.X +
-				srcPoint.Y * srcStride
+		for {
+			dstIndex := dstYComponent + dstPoint.X
+			srcIndex := srcYComponent + srcPoint.X
 			dstData[dstIndex] = srcData[srcIndex]
 
 			srcPoint.X ++
 			if srcPoint.X >= srcBounds.Max.X {
 				srcPoint.X = srcBounds.Min.X
 			}
+
+			dstPoint.X ++
+			if dstPoint.X >= bounds.Max.X {
+				break
+			}
 		}
 
 		srcPoint.Y ++