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>

cmd/gogio/androidbuild.go

@@ -7,8 +7,6 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
-	"golang.org/x/sync/errgroup"
-	"golang.org/x/tools/go/packages"
 	"io"
 	"io/ioutil"
 	"os"
@@ -18,6 +16,9 @@ import (
 	"strconv"
 	"strings"
 	"text/template"
+
+	"golang.org/x/sync/errgroup"
+	"golang.org/x/tools/go/packages"
 )
 
 type androidTools struct {