[dxvk] Implement native discardBuffer function in the backend

This may be more efficient because it avoids renaming the buffer
in case it can be used without inserting additional barriers.

src/d3d11/d3d11_context.cpp

@@ -2728,8 +2728,7 @@ namespace dxvk {
   void D3D11DeviceContext::DiscardBuffer(
           D3D11Buffer*                      pBuffer) {
     EmitCs([cBuffer = pBuffer->GetBuffer()] (DxvkContext* ctx) {
-      ctx->invalidateBuffer(cBuffer,
-        cBuffer->allocPhysicalSlice());
+      ctx->discardBuffer(cBuffer);
     });
   }
 

src/dxvk/dxvk_context.cpp

@@ -941,6 +941,17 @@ namespace dxvk {
   }
   
   
+  void DxvkContext::discardBuffer(
+    const Rc<DxvkBuffer>&       buffer) {
+    DxvkAccessFlags accessFlags(
+      DxvkAccess::Read,
+      DxvkAccess::Write);
+    
+    if (m_barriers.isBufferDirty(buffer->slice(), accessFlags))
+      this->invalidateBuffer(buffer, buffer->allocPhysicalSlice());
+  }
+
+
   void DxvkContext::dispatch(
           uint32_t x,
           uint32_t y,

src/dxvk/dxvk_context.h

@@ -322,6 +322,18 @@ namespace dxvk {
             VkOffset3D            srcOffset,
             VkExtent3D            srcExtent);
     
+    /**
+     * \brief Discards a buffer
+     * 
+     * Renames the buffer in case it is currently
+     * used by the GPU in order to avoid having to
+     * insert barriers before future commands using
+     * the buffer.
+     * \param [in] buffer The buffer to discard
+     */
+    void discardBuffer(
+      const Rc<DxvkBuffer>&       buffer);
+    
     /**
      * \brief Starts compute jobs
      *