Fix crash during scene transition due to invalid camera

[rh101]

Scene can now be visited correctly with valid cameras being set for child nodes when rendering outside of normal render loop.

Which branch your pull-request should merge into?

• 2.1: BugFixes and Improvements for Current LTS branch

Describe your changes

Currently Scene::visit() is only used to draw a scene when rendering to a RenderTexture. The issue with this is that there is no valid camera set for use by the child nodes, so if they access the camera during a call to Node::visit(...), a crash will occur. An example is FastTMXLayer::draw accessing the camera.

The fix in this PR allows the Scene to override visit(), and ensure that the camera is set correctly for all child nodes in that scene.

Scene::visit() is only ever called when attempting to render to a RenderTexture, so the changes in this PR should have no impact on any other functionality in Axmol.

Issue ticket number and link

Crash issue is described in #1663

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