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_darwin.go

@@ -3,7 +3,6 @@
 package headless
 
 import (
-	"gioui.org/app/internal/glimpl"
 	"gioui.org/gpu/backend"
 	"gioui.org/gpu/gl"
 
@@ -24,14 +23,13 @@ __attribute__ ((visibility ("hidden"))) void gio_headless_prepareContext(CFTypeR
 import "C"
 
 type nsContext struct {
-	c        *glimpl.Functions
 	ctx      C.CFTypeRef
 	prepared bool
 }
 
 func newGLContext() (context, error) {
 	ctx := C.gio_headless_newContext()
-	return &nsContext{ctx: ctx, c: new(glimpl.Functions)}, nil
+	return &nsContext{ctx: ctx}, nil
 }
 
 func (c *nsContext) MakeCurrent() error {
@@ -48,11 +46,7 @@ func (c *nsContext) ReleaseCurrent() {
 }
 
 func (c *nsContext) Backend() (backend.Device, error) {
-	return gl.NewBackend(c.c)
-}
-
-func (c *nsContext) Functions() *glimpl.Functions {
-	return c.c
+	return gl.NewBackend(nil)
 }
 
 func (d *nsContext) Release() {

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() {