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_darwin.go

@@ -0,0 +1,55 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package headless
+
+import (
+	"gioui.org/gpu"
+	_ "gioui.org/internal/cocoainit"
+)
+
+/*
+#cgo CFLAGS: -DGL_SILENCE_DEPRECATION -Werror -Wno-deprecated-declarations -fmodules -fobjc-arc -x objective-c
+
+#include <CoreFoundation/CoreFoundation.h>
+
+__attribute__ ((visibility ("hidden"))) CFTypeRef gio_headless_newContext(void);
+__attribute__ ((visibility ("hidden"))) void gio_headless_releaseContext(CFTypeRef ctxRef);
+__attribute__ ((visibility ("hidden"))) void gio_headless_clearCurrentContext(CFTypeRef ctxRef);
+__attribute__ ((visibility ("hidden"))) void gio_headless_makeCurrentContext(CFTypeRef ctxRef);
+__attribute__ ((visibility ("hidden"))) void gio_headless_prepareContext(CFTypeRef ctxRef);
+*/
+import "C"
+
+type nsContext struct {
+	ctx      C.CFTypeRef
+	prepared bool
+}
+
+func newGLContext() (context, error) {
+	ctx := C.gio_headless_newContext()
+	return &nsContext{ctx: ctx}, nil
+}
+
+func (c *nsContext) API() gpu.API {
+	return gpu.OpenGL{}
+}
+
+func (c *nsContext) MakeCurrent() error {
+	C.gio_headless_makeCurrentContext(c.ctx)
+	if !c.prepared {
+		C.gio_headless_prepareContext(c.ctx)
+		c.prepared = true
+	}
+	return nil
+}
+
+func (c *nsContext) ReleaseCurrent() {
+	C.gio_headless_clearCurrentContext(c.ctx)
+}
+
+func (d *nsContext) Release() {
+	if d.ctx != 0 {
+		C.gio_headless_releaseContext(d.ctx)
+		d.ctx = 0
+	}
+}