[dxbc] Declare vertex positions as invariant

Otherwise, games relying on different vertex shaders to produce
the same numerical results may suffer from Z-fighting issues.

Fixes #1364.

src/dxbc/dxbc_compiler.cpp

@@ -740,6 +740,11 @@ namespace dxvk {
         // Add index decoration for potential dual-source blending
         if (m_programInfo.type() == DxbcProgramType::PixelShader)
           m_module.decorateIndex(varId, 0);
+
+        // Declare vertex positions in all stages as invariant, even if
+        // this is not the last stage, to help with potential Z fighting.
+        if (sv == DxbcSystemValue::Position && m_moduleInfo.options.invariantPosition)
+          m_module.decorate(varId, spv::DecorationInvariant);
       }
       
       m_oRegs.at(regIdx) = { regType, varId };

src/dxbc/dxbc_options.cpp

@@ -41,6 +41,10 @@ namespace dxvk {
     zeroInitWorkgroupMemory  = options.zeroInitWorkgroupMemory;
     dynamicIndexedConstantBufferAsSsbo = options.constantBufferRangeCheck;
     
+    // Don't declare vertex positions as invariant on Nvidia since it
+    // may break certain games, such as Shadow of the Tomb Raider.
+    invariantPosition = !adapter->matchesDriver(DxvkGpuVendor::Nvidia, VK_DRIVER_ID_NVIDIA_PROPRIETARY_KHR, 0, 0);
+
     // Disable early discard on RADV (with LLVM) due to GPU hangs
     // Disable early discard on Nvidia because it may hurt performance
     bool isRadvAco = std::string(devInfo.core.properties.deviceName).find("RADV/ACO") != std::string::npos;

src/dxbc/dxbc_options.h

@@ -42,6 +42,9 @@ namespace dxvk {
     /// Clear thread-group shared memory to zero
     bool zeroInitWorkgroupMemory = false;
 
+    /// Declare vertex positions as invariant
+    bool invariantPosition = false;
+
     /// Minimum storage buffer alignment
     VkDeviceSize minSsboAlignment = 0;
   };