Smoke test using DirectX windowless software driver

[jstine35]

• Implement software windowless mode into native engine's DX11 driver
• Add run step to AppVeyor to verify game engine boots and runs without error.

This smoke test will verify at a minimum that resources required to boot the engine are present and loadable. Additional tests can be added later.

Additional Tests (stretch goals)

• Log scrubbing for warnings or errors?
• Verify some output using ImageMagick or such

This stretch goal list is not meant to be part of this task. It should be better defined and converted into its own tracking issue after the software windowless mode and Appveyor integration are complete.

[jstine35]

Submitted PR #111 to provide DX11 windowless operation.