Inconsistent naming related to RenderTarget/RenderTargetLayer between SFML and SDL binding

[Laguna1989]

Needs to be analyzed and named consistently. SDL and SFML work very differently under the hood, so this might cause some ripples throughout the jamtemplate

[Laguna1989]

One starting point might be the GfxImpl class.

• SFML calls m_window.createRenderTarget() (which returns a RenderTargetLayer, which is a sf::RenderTexture) in GfxImpl::createZLayer()
• SDL calls m_window.createRenderTarget() (which returns a RenderTargetLayer, which is a SDLRenderer) in the constructor.

Basically RenderTargetLayer means different things in SDL and SFML binding. This is inconsistent and should be reworked.