The test scenes can now supply their own optional base (background)
color. In the with_winit example, we also allow the user to provide a
base color as a CLI option. The precedence is as follows:
1. Use the color from the CLI options, if any.
2. Otherwise use the scene provided base color, if any.
3. Otherwise default to black.
The texture and surface render API now takes render-time parameters
(such as clear color, target width/height) as a RenderParams struct.
The examples have been updated to demonstrate this. The with_winit
example now accepts a clear color as a command line option.