app/headless: prepare OpenGL context on macOS

The macOS backend uses a desktop OpenGL context, not a OpenGL ES
context. The main difference is that sRGB have to be enabled and
a vertex buffer must be bound.

Do that and fix Window.Screenshot for scenes more complex than a
glClear.

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

app/headless/headless_darwin.go

@@ -13,8 +13,9 @@ import "gioui.org/app/internal/gl"
 import "C"
 
 type nsContext struct {
-	c   *gl.Functions
-	ctx C.CFTypeRef
+	c        *gl.Functions
+	ctx      C.CFTypeRef
+	prepared bool
 }
 
 func newContext() (context, error) {
@@ -24,6 +25,10 @@ func newContext() (context, error) {
 
 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
 }