gpu,gpu/backend: implement generic backend.NewDevice

NewDevice creates a Device given an API, which is the necessary GPU
resources for a backend.

Convert gpu.New to take an API instead of a backend.Device directly.

In turn, this frees us to later unexport the backend package along with
the backend implementations (for now just gioui.org/gpu/gl for OpenGL).
It also allows programs that embed Gio (such as gioui.org/example/glfw)
to freely choose a backend, not just OpenGL.

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

app/headless/headless_darwin.go

@@ -3,9 +3,7 @@
 package headless
 
 import (
-	"gioui.org/gpu/backend"
-	"gioui.org/gpu/gl"
-
+	"gioui.org/gpu"
 	_ "gioui.org/app/internal/cocoainit"
 )
 
@@ -32,6 +30,10 @@ func newGLContext() (context, error) {
 	return &nsContext{ctx: ctx}, nil
 }
 
+func (c *nsContext) API() gpu.API {
+	return gpu.OpenGL{}
+}
+
 func (c *nsContext) MakeCurrent() error {
 	C.gio_headless_makeCurrentContext(c.ctx)
 	if !c.prepared {
@@ -45,10 +47,6 @@ func (c *nsContext) ReleaseCurrent() {
 	C.gio_headless_clearCurrentContext(c.ctx)
 }
 
-func (c *nsContext) Backend() (backend.Device, error) {
-	return gl.NewBackend(nil)
-}
-
 func (d *nsContext) Release() {
 	if d.ctx != 0 {
 		C.gio_headless_releaseContext(d.ctx)