Vendor updates

vendor/github.com/faiface/pixel/pixelgl/window.go

@@ -116,6 +116,7 @@ func NewWindow(cfg WindowConfig) (*Window, error) {
 
 		// enter the OpenGL context
 		w.begin()
+		glhf.Init()
 		w.end()
 
 		return nil
@@ -362,7 +363,6 @@ func (w *Window) CursorVisible() bool {
 func (w *Window) begin() {
 	if currWin != w {
 		w.window.MakeContextCurrent()
-		glhf.Init()
 		currWin = w
 	}
 }
@@ -424,3 +424,8 @@ func (w *Window) Clear(c color.Color) {
 func (w *Window) Color(at pixel.Vec) pixel.RGBA {
 	return w.canvas.Color(at)
 }
+
+// Canvas returns the window's underlying Canvas
+func (w *Window) Canvas() *Canvas {
+	return w.canvas
+}