Occasional background corruption, blank background on macOS

[cxong]

No description provided.

[cxong]

It turns out that the texture refactoring (#440 ) has broken the second window render. According to this thread https://gamedev.stackexchange.com/q/136882/26250 SDL2 doesn't support rendering a single texture to multiple renderers - that is, textures are associated with a single renderer. To render th multiple renderers, we would need to create a texture for each renderer, or render to a target texture and then copy it across to the other renderers.

Will defer this issue to #558