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_js.go

@@ -6,8 +6,7 @@ import (
 	"errors"
 	"syscall/js"
 
-	"gioui.org/gpu/backend"
-	"gioui.org/gpu/gl"
+	"gioui.org/gpu"
 	"gioui.org/internal/glimpl"
 )
 
@@ -31,8 +30,8 @@ func newGLContext() (context, error) {
 	return c, nil
 }
 
-func (c *jsContext) Backend() (backend.Device, error) {
-	return gl.NewBackend(glimpl.Context(c.ctx))
+func (c *jsContext) API() gpu.API {
+	return gpu.OpenGL{Context: glimpl.Context(c.ctx)}
 }
 
 func (c *jsContext) Release() {