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>

gpu/backend/api.go

@@ -0,0 +1,56 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package backend
+
+import (
+	"fmt"
+	"unsafe"
+
+	"gioui.org/internal/glimpl"
+)
+
+// See gpu/api.go for documentation for the API types
+
+type API interface {
+	implementsAPI()
+}
+
+type OpenGL struct {
+	// Context contains the WebGL context for WebAssembly platforms. It is
+	// empty for all other platforms; an OpenGL context is assumed current when
+	// calling NewDevice.
+	Context glimpl.Context
+}
+
+type Direct3D11 struct {
+	// Device contains a *ID3D11Device.
+	Device unsafe.Pointer
+}
+
+// API specific device constructors.
+var (
+	NewOpenGLDevice     func(api OpenGL) (Device, error)
+	NewDirect3D11Device func(api Direct3D11) (Device, error)
+)
+
+// NewDevice creates a new Device given the api.
+//
+// Note that the device does not assume ownership of the resources contained in
+// api; the caller must ensure the resources are valid until the device is
+// released.
+func NewDevice(api API) (Device, error) {
+	switch api := api.(type) {
+	case OpenGL:
+		if NewOpenGLDevice != nil {
+			return NewOpenGLDevice(api)
+		}
+	case Direct3D11:
+		if NewDirect3D11Device != nil {
+			return NewDirect3D11Device(api)
+		}
+	}
+	return nil, fmt.Errorf("backend: no backend available for the API %T", api)
+}
+
+func (OpenGL) implementsAPI()     {}
+func (Direct3D11) implementsAPI() {}