Fixed bug 1622 - SDL_RenderSetViewport with empty SDL_Rect raises wrong error for OpenGL rendering backend

It's now legal to set an empty viewport rect - it will prevent any rendering.

Also added an API to query the output size: SDL_GetRendererOutputSize()
When the window is resized, the viewport is automatically reset.
This resolves lots of confusion around resizable windows.  Most people don't expect a viewport to be implicitly set when the renderer is created and then not to be reset to the window size if the window is resized.

Added common test command line parameters --logical WxH and --scale N to test the render logical size and scaling APIs.