Memory leaks in renderers and canvases cache maps

[pymq]

Describe the bug:

Once created CanvasObject and its renderer are never freed. Two caches are stored here:
internal/driver/glfw/driver.go - canvases
internal/cache/widget.go - renderers
As I can see from code, they are only cleared on window close and only objects that are still in use by that window.
The current workaround is to reuse all widgets but it's hard to do in dynamic interfaces.

To Reproduce:

Continuously create new widgets, for instance in a long running applications.

Example code:

package main

import (
	"time"

	"fyne.io/fyne/app"
	"fyne.io/fyne/widget"
)

func main() {
	a := app.New()
	w := a.NewWindow("test")

	setContent := func() {
		box := widget.NewVBox(
			widget.NewEntry(), widget.NewEntry(),
			widget.NewButton("submit", nil),
		)
		w.SetContent(widget.NewGroup("group", box))
	}

	setContent()
	go func() {
		for range time.NewTicker(100 * time.Millisecond).C {
			setContent()
		}
	}()

	w.CenterOnScreen()
	w.ShowAndRun()
}

Device:

• Go version: 1.13.8
• Fyne version: develop, 84bca6e

[fpabl0]

Fixed on develop.