Revert "cocoa: re-enable double buffering"

Enabling double buffering fixed some graphical glitches when entering
fullscreen, but it also caused a fullscreen performance regression. We
decided that the glitches were preferable to the performance regression.

This reverts commit cee764849e.

video/out/cocoa_common.m

@@ -811,8 +811,6 @@ 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

@@ -72,7 +72,6 @@ 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
@@ -156,7 +155,10 @@ static void cocoa_uninit(struct ra_ctx *ctx)
 
 static void cocoa_swap_buffers(struct ra_ctx *ctx)
 {
+    struct priv *p = ctx->priv;
+    GL *gl = &p->gl;
     vo_cocoa_swap_buffers(ctx->vo);
+    gl->Flush();
 }
 
 static bool cocoa_init(struct ra_ctx *ctx)