Offscreen invalidate and sizing fix

[gerhardberger]

This PR adds an invalidate method to webContents to manually request a new frame in offscreen mode.

And also fixes the issue that a browser did not resize properly in offscreen mode.

[zeke]

Hi @gerhardberger. What is the use case for this new method?

[gerhardberger]

@zeke, it generates a 'paint' event on-demand with the current frame. This can be used when you need the latest frame but no change happens on the page, e.g. you start rendering to a new place and you want an initial frame.

[zcbenz]

IsOffScreen should be checked here.

[gerhardberger]

I thought about that, but couldn't come up with a proper way to get the atom::api::WebContents in NativeWindow.

But this SetSize call only happens in OSR mode, because in a normal rendering mode having a different size other than the one we wanted to set after we set it should not happen.

[zcbenz]

How about subscribing to window resize events in RenderWidgetHostView? It is provided by NativeWindowObserver.

[gerhardberger]

Great I didn't notice that, thanks! I moved the call to OffscreenRenderWidgetHostView.

[danburzo]

This is great! I was doing a stopPainting()/startPainting() combo to force a paint event.

The use-case (for me, at least) is you want to grab a PNG representation of the page after a certain client event is emitted, and since the Buffer is volatile, I'm not sure how you'd have done it previously (other than triggering a paint event).