cocoa: re-enable double buffering

This causes a performance regression on 10.11 and newer, but the single
buffered method was broken and could cause partially rendered frames to
be presented to the screen.

This reverts 9f30cd8292 and
e543853a7f.

video/out/cocoa_common.m

@@ -811,6 +811,8 @@ void vo_cocoa_swap_buffers(struct vo *vo)
     }
     pthread_mutex_unlock(&s->sync_lock);
 
+    CGLFlushDrawable(s->cgl_ctx);
+
     pthread_mutex_lock(&s->lock);
     s->frame_w = vo->dwidth;
     s->frame_h = vo->dheight;

video/out/opengl/context_cocoa.c

@@ -72,6 +72,7 @@ static CGLError test_gl_version(struct ra_ctx *ctx, CGLOpenGLProfile ver)
         // rejected attribute to preserve the fallback code
         kCGLPFAOpenGLProfile,
         (CGLPixelFormatAttribute) ver,
+        kCGLPFADoubleBuffer,
         kCGLPFAAccelerated,
         kCGLPFAAllowOfflineRenderers,
         // keep this one last to apply the cocoa-force-dedicated-gpu option
@@ -155,9 +156,7 @@ static void cocoa_uninit(struct ra_ctx *ctx)
 
 static void cocoa_swap_buffers(struct ra_ctx *ctx)
 {
-    GL *gl = &p->gl;
     vo_cocoa_swap_buffers(ctx->vo);
-    gl->Flush();
 }
 
 static bool cocoa_init(struct ra_ctx *ctx)