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/backend_test.go

@@ -170,7 +170,7 @@ func newBackend(t *testing.T) backend.Device {
 	if err := ctx.MakeCurrent(); err != nil {
 		t.Fatal(err)
 	}
-	b, err := ctx.Backend()
+	b, err := backend.NewDevice(ctx.API())
 	if err != nil {
 		t.Fatal(err)
 	}

app/headless/headless.go

@@ -25,7 +25,7 @@ type Window struct {
 }
 
 type context interface {
-	Backend() (backend.Device, error)
+	API() gpu.API
 	MakeCurrent() error
 	ReleaseCurrent()
 	Release()
@@ -42,7 +42,8 @@ func NewWindow(width, height int) (*Window, error) {
 		ctx:  ctx,
 	}
 	err = contextDo(ctx, func() error {
-		dev, err := ctx.Backend()
+		api := ctx.API()
+		dev, err := backend.NewDevice(api)
 		if err != nil {
 			return err
 		}
@@ -62,7 +63,7 @@ func NewWindow(width, height int) (*Window, error) {
 			fboTex.Release()
 			return err
 		}
-		gp, err := gpu.New(dev)
+		gp, err := gpu.New(api)
 		if err != nil {
 			fbo.Release()
 			fboTex.Release()

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)

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() {

app/headless/headless_windows.go

@@ -3,8 +3,10 @@
 package headless
 
 import (
+	"unsafe"
+
+	"gioui.org/gpu"
 	"gioui.org/app/internal/d3d11"
-	"gioui.org/gpu/backend"
 )
 
 type d3d11Context struct {
@@ -19,12 +21,8 @@ func newContext() (context, error) {
 	return &d3d11Context{dev: dev}, nil
 }
 
-func (c *d3d11Context) Backend() (backend.Device, error) {
-	backend, err := d3d11.NewBackend(c.dev.Handle)
-	if err != nil {
-		return nil, err
-	}
-	return backend, nil
+func (c *d3d11Context) API() gpu.API {
+	return gpu.Direct3D11{Device: unsafe.Pointer(c.dev.Handle)}
 }
 
 func (c *d3d11Context) MakeCurrent() error {