app/headless: re-implement Screenshot on top of Backend

The new Framebuffer.ReadPixels method is enough to implement
Window.Screenshot. Use that instead of the OpenGL-specific
implementation.

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

app/headless/headless_gl.go

@@ -2,72 +2,6 @@
 
 package headless
 
-import (
-	"fmt"
-	"runtime"
-
-	"gioui.org/app/internal/glimpl"
-	"gioui.org/app/internal/srgb"
-	"gioui.org/gpu"
-	"gioui.org/gpu/gl"
-)
-
-type glContext interface {
-	Functions() *glimpl.Functions
-	Backend() (gpu.Backend, error)
-	MakeCurrent() error
-	ReleaseCurrent()
-	Release()
-}
-
-type glBackend struct {
-	glContext
-	srgb *srgb.FBO
-}
-
 func newContext(width, height int) (backend, error) {
-	glctx, err := newGLContext()
-	if err != nil {
-		return nil, err
-	}
-	// Create the back buffer FBO after locking the thread and making
-	// the context current.
-	runtime.LockOSThread()
-	defer runtime.UnlockOSThread()
-	if err := glctx.MakeCurrent(); err != nil {
-		glctx.Release()
-		return nil, err
-	}
-	defer glctx.ReleaseCurrent()
-	fbo, err := srgb.New(glctx.Functions())
-	if err != nil {
-		glctx.Release()
-		return nil, err
-	}
-	if err := fbo.Refresh(width, height); err != nil {
-		fbo.Release()
-		glctx.Release()
-		return nil, err
-	}
-	return &glBackend{glctx, fbo}, nil
-}
-
-func (b *glBackend) Screenshot(width, height int, pixels []byte) error {
-	if len(pixels) != width*height*4 {
-		panic("unexpected RGBA size")
-	}
-	f := b.Functions()
-	f.ReadPixels(0, 0, width, height, gl.RGBA, gl.UNSIGNED_BYTE, pixels)
-	if glErr := f.GetError(); glErr != gl.NO_ERROR {
-		return fmt.Errorf("glReadPixels failed: %d", glErr)
-	}
-	return nil
-}
-
-func (b *glBackend) Release() {
-	if b.srgb != nil {
-		b.srgb.Release()
-		b.srgb = nil
-	}
-	b.glContext.Release()
+	return newGLContext()
 }