Fixed issue when setting ViewPort multiple times per frame.

[SergioRZMasson]

Overview

This PR fixes #1182, it moves viewport update from been done immediately to be done using the command queue. The way this is currently been done on master, if multiple viewport updates are done in a single frame (like when you can more than one camera been rendered using different viewports) the viewport updates actually end up overriding each other and get lost by the time the command queue starts been processed.

Solution

In order to fix this, I'm proposing moving the setViewPort operation to be performed in the command queue. This way, this operation is going to happen in sync with the other rendering commands like "FrameBuffer::Clear" (which also generates a bgfx::View). I also had to modify BabylonNative::FrameBuffer since in some situations the FrameBuffer::SetViewPort gets called before FrameBuffer::Clear and sometimes it gets called after. In the cases where it gets called after, it is required that we store the clear information in the FrameBuffer object so we can call bgfx::setViewClear and that information does not get lost if a new bgfx::View gets created.

This will also require a PR in Babylon.js to change how NativeEngine calls setViewPort. Babylon.js PR

Extra Information

This functionality started as a request in the forum:
Forum issue
Target web behaviour

[bghgary]

LGTM. We should test XR and other scenarios are still working.

[SergioRZMasson]

Working fine on Android XR.

[SergioRZMasson]

Working fine on IOS XR.