app/headless: re-implement Screenshot on top of Backend

The new Framebuffer.ReadPixels method is enough to implement
Window.Screenshot. Use that instead of the OpenGL-specific
implementation.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/headless/headless_js.go

@@ -16,7 +16,7 @@ type jsContext struct {
 	f   *glimpl.Functions
 }
 
-func newGLContext() (glContext, error) {
+func newGLContext() (backend, error) {
 	version := 2
 	doc := js.Global().Get("document")
 	cnv := doc.Call("createElement", "canvas")