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/internal/window/window.go

@@ -7,7 +7,7 @@ package window
 import (
 	"errors"
 
-	"gioui.org/gpu/backend"
+	"gioui.org/gpu"
 	"gioui.org/io/event"
 	"gioui.org/io/pointer"
 	"gioui.org/io/system"
@@ -33,7 +33,7 @@ type Callbacks interface {
 }
 
 type Context interface {
-	Backend() (backend.Device, error)
+	API() gpu.API
 	Present() error
 	MakeCurrent() error
 	Release()