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/d3d11_windows.go

@@ -3,8 +3,10 @@
 package window
 
 import (
+	"unsafe"
+
+	"gioui.org/gpu"
 	"gioui.org/app/internal/d3d11"
-	"gioui.org/gpu/backend"
 )
 
 type d3d11Context struct {
@@ -34,8 +36,8 @@ func init() {
 	})
 }
 
-func (c *d3d11Context) Backend() (backend.Device, error) {
-	return d3d11.NewBackend(c.dev.Handle)
+func (c *d3d11Context) API() gpu.API {
+	return gpu.Direct3D11{Device: unsafe.Pointer(c.dev.Handle)}
 }
 
 func (c *d3d11Context) Present() error {

app/internal/window/gl_ios.go

@@ -21,8 +21,7 @@ import (
 	"errors"
 	"fmt"
 
-	"gioui.org/gpu/backend"
-	"gioui.org/gpu/gl"
+	"gioui.org/gpu"
 	"gioui.org/internal/glimpl"
 )
 
@@ -56,8 +55,8 @@ func newContext(w *window) (*context, error) {
 	return c, nil
 }
 
-func (c *context) Backend() (backend.Device, error) {
-	return gl.NewBackend(nil)
+func (c *context) API() gpu.API {
+	return gpu.OpenGL{}
 }
 
 func (c *context) Release() {

app/internal/window/gl_js.go

@@ -7,8 +7,7 @@ import (
 	"syscall/js"
 
 	"gioui.org/app/internal/srgb"
-	"gioui.org/gpu/backend"
-	"gioui.org/gpu/gl"
+	"gioui.org/gpu"
 	"gioui.org/internal/glimpl"
 )
 
@@ -39,8 +38,8 @@ func newContext(w *window) (*context, error) {
 	return c, nil
 }
 
-func (c *context) Backend() (backend.Device, error) {
-	return gl.NewBackend(glimpl.Context(c.ctx))
+func (c *context) API() gpu.API {
+	return gpu.OpenGL{Context: glimpl.Context(c.ctx)}
 }
 
 func (c *context) Release() {

app/internal/window/gl_macos.go

@@ -5,8 +5,7 @@
 package window
 
 import (
-	"gioui.org/gpu/backend"
-	"gioui.org/gpu/gl"
+	"gioui.org/gpu"
 	"gioui.org/internal/glimpl"
 )
 
@@ -48,8 +47,8 @@ func newContext(w *window) (*context, error) {
 	return c, nil
 }
 
-func (c *context) Backend() (backend.Device, error) {
-	return gl.NewBackend(nil)
+func (c *context) API() gpu.API {
+	return gpu.OpenGL{}
 }
 
 func (c *context) Release() {

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()