app/headless: remove OpenGL assumptions

To prepare package headless for multiple backends, refactor the common headless
driver to no longer assume an OpenGL context. Instead, introduce a headless
backend type and the OpenGL implementation, glBackend.

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

app/headless/headless_gl.go

@@ -0,0 +1,73 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+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.SRGBFBO
+}
+
+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.NewSRGBFBO(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()
+}