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.

backends/x/event.go

@@ -1,5 +1,6 @@
 package x
 
+import "image"
 import "git.tebibyte.media/sashakoshka/tomo/input"
 import "git.tebibyte.media/sashakoshka/tomo/elements"
 
@@ -47,8 +48,8 @@ func (window *Window) handleExpose (
 	connection *xgbutil.XUtil,
 	event xevent.ExposeEvent,
 ) {
-	_ = window.compressExpose(*event.ExposeEvent)
-	window.redrawChildEntirely()
+	_, region := window.compressExpose(*event.ExposeEvent)
+	window.pushRegion(region)
 }
 
 func (window *Window) handleConfigureNotify (
@@ -234,7 +235,13 @@ func (window *Window) compressExpose (
 	firstEvent xproto.ExposeEvent,
 ) (
 	lastEvent xproto.ExposeEvent,
+	region image.Rectangle,
 ) {
+	region = image.Rect (
+		int(firstEvent.X), int(firstEvent.Y),
+		int(firstEvent.X + firstEvent.Width),
+		int(firstEvent.Y + firstEvent.Height))
+	
 	window.backend.connection.Sync()
 	xevent.Read(window.backend.connection, false)
 	lastEvent = firstEvent
@@ -245,8 +252,12 @@ func (window *Window) compressExpose (
 		typedEvent, ok := untypedEvent.Event.(xproto.ExposeEvent)
 		if !ok { continue }
 
-		// FIXME: union all areas into the last event
 		if firstEvent.Window == typedEvent.Window {
+			region = region.Union (image.Rect (
+				int(typedEvent.X), int(typedEvent.Y),
+				int(typedEvent.X + typedEvent.Width),
+				int(typedEvent.Y + typedEvent.Height)))
+		
 			lastEvent = typedEvent
 			defer func (index int) {
 				xevent.DequeueAt(window.backend.connection, index)

backends/x/window.go

@@ -12,6 +12,7 @@ import "git.tebibyte.media/sashakoshka/tomo/theme"
 import "git.tebibyte.media/sashakoshka/tomo/config"
 import "git.tebibyte.media/sashakoshka/tomo/canvas"
 import "git.tebibyte.media/sashakoshka/tomo/elements"
+// import "runtime/debug"
 
 type Window struct {
 	backend *Backend
@@ -290,6 +291,7 @@ func (window *Window) childDrawCallback (region canvas.Canvas) {
 func (window *Window) paste (canvas canvas.Canvas) (updatedRegion image.Rectangle) {
 	data, stride := canvas.Buffer()
 	bounds := canvas.Bounds().Intersect(window.xCanvas.Bounds())
+	// debug.PrintStack()
 	for x := bounds.Min.X; x < bounds.Max.X; x ++ {
 	for y := bounds.Min.Y; y < bounds.Max.Y; y ++ {
 		rgba := data[x + y * stride]