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/headless/headless_js.go

@@ -6,14 +6,13 @@ import (
 	"errors"
 	"syscall/js"
 
-	"gioui.org/app/internal/glimpl"
 	"gioui.org/gpu/backend"
 	"gioui.org/gpu/gl"
+	"gioui.org/internal/glimpl"
 )
 
 type jsContext struct {
 	ctx js.Value
-	f   *glimpl.Functions
 }
 
 func newGLContext() (context, error) {
@@ -26,23 +25,14 @@ func newGLContext() (context, error) {
 	if ctx.IsNull() {
 		return nil, errors.New("headless: webgl is not supported")
 	}
-	f := &glimpl.Functions{Ctx: ctx}
-	if err := f.Init(); err != nil {
-		return nil, err
-	}
 	c := &jsContext{
 		ctx: ctx,
-		f:   f,
 	}
 	return c, nil
 }
 
 func (c *jsContext) Backend() (backend.Device, error) {
-	return gl.NewBackend(c.f)
-}
-
-func (c *jsContext) Functions() *glimpl.Functions {
-	return c.f
+	return gl.NewBackend(glimpl.Context(c.ctx))
 }
 
 func (c *jsContext) Release() {