Use at least one BackBuffer in D3D8DeviceEx

Gets Red Faction going up to the game menu and mirrors what D3D9 does. I *think* I'm doing the right thing here, but please treat all my PRs as coming from a monkey with a wrench inside of a nuclear reactor.

This was discovered while investigating a crash in IDirect3DDevice8::GetBackBuffer. Red Faction will create a IDirect3DDevice8 with BackBufferCount = 0, then call GetBackBuffer for iBackBuffer = 0, which would trigger a crash.

src/d3d8/d3d8_device.cpp

@@ -69,6 +69,9 @@ namespace dxvk {
       D3DRS_ZVISIBLE,
       D3DRS_PATCHSEGMENTS
     );
+
+    // Mirrors how D3D9 handles the BackBufferCount
+    m_presentParams.BackBufferCount = std::max(m_presentParams.BackBufferCount, 1u);
 
     m_textures.fill(nullptr);
     m_streams.fill(D3D8VBO());
@@ -547,4 +550,4 @@ namespace dxvk {
     return pPixelShader->GetFunction(pData, &SizeOfData);
   }
 
-} // namespace dxvk
+} // namespace dxvk