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_ios.m

@@ -27,3 +27,6 @@ void gio_headless_makeCurrentContext(CFTypeRef ctxRef) {
 	EAGLContext *ctx = (__bridge EAGLContext *)ctxRef;
 	[EAGLContext setCurrentContext:ctx];
 }
+
+void gio_headless_prepareContext(CFTypeRef ctxRef) {
+}