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/gl_macos.go

@@ -6,6 +6,7 @@ package window
 
 import (
 	"gioui.org/app/internal/gl"
+	"gioui.org/app/internal/gl/impl"
 )
 
 /*
@@ -18,7 +19,7 @@ import (
 import "C"
 
 type context struct {
-	c    *gl.Functions
+	c    gl.Functions
 	ctx  C.CFTypeRef
 	view C.CFTypeRef
 }
@@ -34,13 +35,13 @@ func newContext(w *window) (*context, error) {
 	ctx := C.gio_contextForView(view)
 	c := &context{
 		ctx:  ctx,
-		c:    new(gl.Functions),
+		c:    new(impl.Functions),
 		view: view,
 	}
 	return c, nil
 }
 
-func (c *context) Functions() *gl.Functions {
+func (c *context) Functions() gl.Functions {
 	return c.c
 }