Small RSGetViewports fix

[K0bin]

Closes #1116

In D3D11DeviceContext::RSGetViewports, D3D11 only sets pNumViewports if pViewports is null.
I've verified this with Windows D3D11. Wine also does it that way.

The behavior for RSGetScissorRects is the same.

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MyGUI (a gui library that Scrap Mechanic uses) has a useless assert that only checks D3D11 runtime behavior.

UINT numViewports = 0;
D3D11_VIEWPORT viewports[8];
mpD3DContext->RSGetViewports(&numViewports, viewports);
MYGUI_PLATFORM_ASSERT(numViewports == 0, getClassTypeName() << " 0 viewport sets");

It does have a viewport at the point this gets executed but because it passes a valid pointer as pViewports, the count doesnt get changed.

https://github.com/MyGUI/mygui/blob/master/Platforms/DirectX11/DirectX11Platform/src/MyGUI_DirectX11RenderManager.cpp#L122

The MyGUI demos run fine with this PR.

[misyltoad]

else if (pNumViewports != nullptr) perhaps? Not in our best interests to crash from user data.

[K0bin]

Windows D3D11 also just crashes when you pass null for that.

[doitsujin]

Merged manually.