Reloading engine does not reset static variables

[iche033]

Description

See #1280 (review), specifically:

Checking with nm -SlC lib/libgz-rendering-ogre2.dylib | grep -F ' b ' | v, I see the following non-const variables that will not be reinitialized:

gz::rendering::v11::Ogre2RenderTarget::TargetFSAA(unsigned char)::ogre2FSAAWarn
gz::rendering::v11::Ogre2DynamicRenderable::CreateDynamicMesh()::dynamicRenderableId
gz::rendering::v11::Ogre2GaussianNoisePass::CreateRenderPass()::gaussianNodeCounter
gz::rendering::v11::Ogre2DepthGaussianNoisePass::CreateRenderPass()::gaussianDepthNodeCounter

and the following static initializers that affect singletons from https://github.com/gazebosim/gz-rendering/blob/15a0455307f002e9a5e1415198ebbd7ab5731c1b/include/gz/rendering/RenderPassSystem.hh#L97> :

global_Ogre2LensFlarePassFactory
global_Ogre2GaussianNoisePassFactory

We may have to reset these variables when engine is unloaded and reloaded.