Expose a pixelgl.Window.SwapBuffers method (that doesn't poll events)

[snargleplax]

I'd like to be able to call the part of pixelgl.Window.Update that swaps buffers without calling the part that polls input. Since UpdateInput has already been broken out and exported (cf. #78), this symmetric case seems like a reasonable extension of the same principle.

My use case is an architecture that polls input via UpdateInput and JustPressed in a separate goroutine from the graphics refresh loop. Because the graphics goroutine polls input as a side-effect of its call to Update, the input handling goroutine's call to JustPressed doesn't trigger reliably -- because, I surmise, the two are racing and sometimes the graphics update "swallows" the first-time input. Regular Pressed works, but then I'd have to roll my own edge detection.

I've tested this change locally (since it's trivial to make) and it seems to work fine:

diff --git a/pixelgl/window.go b/pixelgl/window.go
index 10c16ce..14ce2d9 100644
--- a/pixelgl/window.go
+++ b/pixelgl/window.go
@@ -173,6 +173,11 @@ func (w *Window) Destroy() {

 // Update swaps buffers and polls events. Call this method at the end of each frame.
 func (w *Window) Update() {
+       w.SwapBuffers()
+       w.UpdateInput()
+}
+
+func (w *Window) SwapBuffers() {
        mainthread.Call(func() {
                _, _, oldW, oldH := intBounds(w.bounds)
                newW, newH := w.window.GetSize()
@@ -207,8 +212,6 @@ func (w *Window) Update() {
                w.window.SwapBuffers()
                w.end()
        })
-
-       w.UpdateInput()
 }

 // SetClosed sets the closed flag of the Window.

I'll submit a PR with this.

Thanks for this library, been playing with it about a week now and it's been great.

[delp]

Addressed by #219
Thanks for the change!