gpu/headless: move package app/headless

Package headless is more about rendering that windows. Move it
accordingly.

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

gpu/headless/headless_js.go

@@ -0,0 +1,45 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package headless
+
+import (
+	"errors"
+	"syscall/js"
+
+	"gioui.org/gpu"
+	"gioui.org/internal/glimpl"
+)
+
+type jsContext struct {
+	ctx js.Value
+}
+
+func newGLContext() (context, error) {
+	doc := js.Global().Get("document")
+	cnv := doc.Call("createElement", "canvas")
+	ctx := cnv.Call("getContext", "webgl2")
+	if ctx.IsNull() {
+		ctx = cnv.Call("getContext", "webgl")
+	}
+	if ctx.IsNull() {
+		return nil, errors.New("headless: webgl is not supported")
+	}
+	c := &jsContext{
+		ctx: ctx,
+	}
+	return c, nil
+}
+
+func (c *jsContext) API() gpu.API {
+	return gpu.OpenGL{Context: glimpl.Context(c.ctx)}
+}
+
+func (c *jsContext) Release() {
+}
+
+func (c *jsContext) ReleaseCurrent() {
+}
+
+func (c *jsContext) MakeCurrent() error {
+	return nil
+}