[3.0] direct3d11: allow using the winrt swapchain/d3dcontext hack in non UWP builds

Since we don't have GPU callbacks in 3.0 we can still allow rendering as we used to in the UWP app for regular apps that want rendering in their swapchain.

Also add a sample app to demonstrate how to use this.

This hack still exists in 4.0 in UWP builds but should be removed.

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v2:

• add missing support for GUID_CONTEXT_MUTEX in the sample app

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v3:

• add some likely/unlikely to show which is the normal code path (no external rendering).
• move the UWP GetRect in a separate patch to show the code hasn't changed.