Cache the canvas lookup for huge speedup

We could be calling this 100s or 1000s of times if the entire text grid was refreshed
(up to twice for each cell, possibly faster than refresh rate because this in the event thread not draw).

Result is huge boost (see fyneterm usage for example)

widget/textgrid.go

@@ -374,7 +374,7 @@ func (t *textGridRenderer) setCellRune(str rune, pos int, style, rowStyle TextGr
 	if text.Text != newStr || text.Color != fg {
 		text.Text = newStr
 		text.Color = fg
-		canvas.Refresh(text)
+		t.refresh(text)
 	}
 
 	rect := t.objects[pos*2].(*canvas.Rectangle)
@@ -386,7 +386,7 @@ func (t *textGridRenderer) setCellRune(str rune, pos int, style, rowStyle TextGr
 	}
 	if rect.FillColor != bg {
 		rect.FillColor = bg
-		canvas.Refresh(rect)
+		t.refresh(rect)
 	}
 }
 
@@ -527,6 +527,11 @@ func (t *textGridRenderer) MinSize() fyne.Size {
 }
 
 func (t *textGridRenderer) Refresh() {
+	// we may be on a new canvas, so just update it to be sure
+	if fyne.CurrentApp() != nil && fyne.CurrentApp().Driver() != nil {
+		t.current = fyne.CurrentApp().Driver().CanvasForObject(t.text)
+	}
+
 	// theme could change text size
 	t.updateCellSize()
 
@@ -545,6 +550,21 @@ func (t *textGridRenderer) Objects() []fyne.CanvasObject {
 func (t *textGridRenderer) Destroy() {
 }
 
+func (t *textGridRenderer) refresh(obj fyne.CanvasObject) {
+	if t.current == nil {
+		if fyne.CurrentApp() != nil && fyne.CurrentApp().Driver() != nil {
+			// cache canvas for this widget, so we don't look it up many times for every cell/row refresh!
+			t.current = fyne.CurrentApp().Driver().CanvasForObject(t.text)
+		}
+
+		if t.current == nil {
+			return // not yet set up perhaps?
+		}
+	}
+
+	t.current.Refresh(obj)
+}
+
 func (t *textGridRenderer) updateCellSize() {
 	size := fyne.MeasureText("M", theme.TextSize(), fyne.TextStyle{Monospace: true})