cocoa-cb: fix hwdec when drawing off-screen

drawing off-screen failed because we didn't have a valid context. the
problem is we force off-screen drawing because the CAOpenGLLayer refuses
to draw anything while being off-screen. set the current context before
starting to draw anything off-screen.

Fixes #5530

video/out/cocoa-cb/video_layer.swift

@@ -198,6 +198,7 @@ class VideoLayer: CAOpenGLLayer {
             if !cocoaCB.window.occlusionState.contains(.visible) &&
                 neededFlips > 1 && canDrawOffScreen
             {
+                CGLSetCurrentContext(cglContext!)
                 draw(cglContext!)
                 display()
             } else {