[dxvk] Set empty scissor rect when the app requests empty viewport

Since we cannot set the viewport size to zero, we should set an
empty scissor rect so that rasterization is still effectively
disabled for the given viewport index.

Fixes #813, #957.

src/dxvk/dxvk_context.cpp

@@ -1895,10 +1895,14 @@ namespace dxvk {
       m_state.vp.scissorRects[i] = scissorRects[i];
       
       // Vulkan viewports are not allowed to have a width or
-      // height of zero, so we fall back to a dummy viewport.
+      // height of zero, so we fall back to a dummy viewport
+      // and instead set an empty scissor rect, which is legal.
       if (viewports[i].width == 0.0f || viewports[i].height == 0.0f) {
         m_state.vp.viewports[i] = VkViewport {
           0.0f, 0.0f, 1.0f, 1.0f, 0.0f, 1.0f };
+        m_state.vp.scissorRects[i] = VkRect2D {
+          VkOffset2D { 0, 0 },
+          VkExtent2D { 0, 0 } };
       }
     }