app/internal/gl/impl: extract platform dependent opengl bindings

We'd like to support Gio using a different renderer binding than
the builtin. A first step is to define the Functions interface
in package gl, and extract the concrete implementations to a
separate package.

Updates #26

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

app/internal/window/window.go

@@ -32,7 +32,7 @@ type Callbacks interface {
 }
 
 type Context interface {
-	Functions() *gl.Functions
+	Functions() gl.Functions
 	Present() error
 	MakeCurrent() error
 	Release()