gpu,gpu/gl: introduce Backend

A recent change made the OpenGL functions an interface of the functions
required for the implementation of GPU, a renderer for Gio operations.
That allowed for running Gio on external systems where OpenGL is
available.

However, to allow for non-OpenGL flavored backends such as Vulkan,
Metal and Direct3D, this change introduces Backend for the high-level
operations required by GPU. This change also adds a concrete backend
to package gl.

Type Backend is a first cut heavily based on OpenGL. Future changes will add
more backends, where the Backend interface quite possibly will need refinement.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/headless/headless.go

@@ -51,7 +51,13 @@ func NewWindow(width, height int) (*Window, error) {
 			ctx.Release()
 			return err
 		}
-		gpu, err := gpu.New(f)
+		backend, err := gl.NewBackend(f)
+		if err != nil {
+			fbo.Release()
+			ctx.Release()
+			return err
+		}
+		gpu, err := gpu.New(backend)
 		if err != nil {
 			fbo.Release()
 			ctx.Release()

app/internal/egl/egl_windows.go

@@ -11,7 +11,7 @@ import (
 	syscall "golang.org/x/sys/windows"
 
 	"gioui.org/app/internal/glimpl"
-	"gioui.org/gpu/gl"
+	gunsafe "gioui.org/internal/unsafe"
 )
 
 type (
@@ -157,7 +157,7 @@ func eglTerminate(disp _EGLDisplay) bool {
 
 func eglQueryString(disp _EGLDisplay, name _EGLint) string {
 	r, _, _ := _eglQueryString.Call(uintptr(disp), uintptr(name))
-	return gl.GoString(gl.SliceOf(r))
+	return gunsafe.GoString(gunsafe.SliceOf(r))
 }
 
 // issue34474KeepAlive calls runtime.KeepAlive as a

app/internal/glimpl/gl_windows.go

@@ -11,6 +11,7 @@ import (
 	"golang.org/x/sys/windows"
 
 	"gioui.org/gpu/gl"
+	gunsafe "gioui.org/internal/unsafe"
 )
 
 var (
@@ -290,7 +291,7 @@ func (c *Functions) GetShaderInfoLog(s gl.Shader) string {
 }
 func (c *Functions) GetString(pname gl.Enum) string {
 	s, _, _ := syscall.Syscall(_glGetString.Addr(), 1, uintptr(pname), 0, 0)
-	return gl.GoString(gl.SliceOf(s))
+	return gunsafe.GoString(gunsafe.SliceOf(s))
 }
 func (c *Functions) GetUniformLocation(p gl.Program, name string) gl.Uniform {
 	cname := cString(name)

app/loop.go

@@ -8,6 +8,7 @@ import (
 
 	"gioui.org/app/internal/window"
 	"gioui.org/gpu"
+	"gioui.org/gpu/gl"
 	"gioui.org/op"
 )
 
@@ -66,7 +67,12 @@ func (l *renderLoop) renderLoop(glctx window.Context) error {
 			initErr <- err
 			return
 		}
-		g, err := gpu.New(glctx.Functions())
+		ctx, err := gl.NewBackend(glctx.Functions())
+		if err != nil {
+			initErr <- err
+			return
+		}
+		g, err := gpu.New(ctx)
 		if err != nil {
 			initErr <- err
 			return