gpu/gl: remove OpenGL functions parameter from NewBackend

As a consequence, most API is gone from gpu/gl, and embedding Gio in
foreign frameworks don't need to provide an OpenGL implementation.

The next change simplifies the GLFW embedding example accordingly.

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

app/internal/egl/egl_windows.go

@@ -10,7 +10,7 @@ import (
 
 	syscall "golang.org/x/sys/windows"
 
-	"gioui.org/app/internal/glimpl"
+	"gioui.org/internal/glimpl"
 	gunsafe "gioui.org/internal/unsafe"
 )