Fix window resizing in SDL+DX11 example

[joeslay]

Make the dx11 example implementation resize the window in a sane way, i.e. not the stretching/scaling the initial render to fit the new window size (current behaviour).

This is what happens if I resize initial window currently: mouse doesn't hit detect on the controls properly (mouse not shown, but it's actually hovering in the blue), scaling makes it unreadable.

with the new code:

.

[ocornut]

Thank you @joeslay, it was weirdly overlooked because the other DX11 example are doing that in their WndProc handler. I pushed an additional commit to compare the WindowId (so it works with multi-viewports) and changed the code to use CleanupRenderTarget() + ResizeBuffers() + CreateRenderTarget() as with the other DX11 example.

I removed g_pd3dDeviceContext->OMSetRenderTargets(0, 0, 0); because even though it makes senses the other DX11 example never did that, tried to enable D3D11_CREATE_DEVICE_DEBUG and didn't get any issue so I assume it isn't required.

Thanks again!