app/headless: implement headless windows

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

app/headless/headless_js.go

@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package headless
+
+import (
+	"errors"
+	"syscall/js"
+
+	"gioui.org/app/internal/gl"
+)
+
+type jsContext struct {
+	ctx js.Value
+	f   *gl.Functions
+}
+
+func newContext() (*jsContext, error) {
+	version := 2
+	doc := js.Global().Get("document")
+	cnv := doc.Call("createElement", "canvas")
+	ctx := cnv.Call("getContext", "webgl2")
+	if ctx.IsNull() {
+		version = 1
+		ctx = cnv.Call("getContext", "webgl")
+	}
+	if ctx.IsNull() {
+		return nil, errors.New("headless: webgl is not supported")
+	}
+	f := &gl.Functions{Ctx: ctx}
+	if err := f.Init(version); err != nil {
+		return nil, err
+	}
+	c := &jsContext{
+		ctx: ctx,
+		f:   f,
+	}
+	return c, nil
+}
+
+func (c *jsContext) Functions() *gl.Functions {
+	return c.f
+}
+
+func (c *jsContext) Release() {
+}
+
+func (c *jsContext) ReleaseCurrent() {
+}
+
+func (c *jsContext) MakeCurrent() error {
+	return nil
+}