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_macos.m

@@ -5,6 +5,7 @@
 @import AppKit;
 @import OpenGL;
 @import OpenGL.GL;
+@import OpenGL.GL3;
 
 #include <CoreFoundation/CoreFoundation.h>
 #include "headless_darwin.h"
@@ -40,6 +41,13 @@ void gio_headless_clearCurrentContext(CFTypeRef ctxRef) {
 void gio_headless_makeCurrentContext(CFTypeRef ctxRef) {
 	NSOpenGLContext *ctx = (__bridge NSOpenGLContext *)ctxRef;
 	[ctx makeCurrentContext];
-	glEnable(GL_FRAMEBUFFER_SRGB);
 	CGLLockContext([ctx CGLContextObj]);
 }
+
+void gio_headless_prepareContext(CFTypeRef ctxRef) {
+	// Bind a default VBA to emulate OpenGL ES 2.
+	GLuint defVBA;
+	glGenVertexArrays(1, &defVBA);
+	glBindVertexArray(defVBA);
+	glEnable(GL_FRAMEBUFFER_SRGB);
+}