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_egl.go

@@ -8,6 +8,6 @@ import (
 	"gioui.org/app/internal/egl"
 )
 
-func newGLContext() (glContext, error) {
+func newGLContext() (backend, error) {
 	return egl.NewContext(egl.EGL_DEFAULT_DISPLAY)
 }