[dxvk] Add function to insert framebuffer read-back barriers

Required for some D3D9 content.

src/dxvk/dxvk_context.cpp

@@ -1586,6 +1586,17 @@ namespace dxvk {
   }
 
 
+  void DxvkContext::emitRenderTargetReadbackBarrier() {
+    if (m_flags.test(DxvkContextFlag::GpRenderPassBound)) {
+      emitMemoryBarrier(VK_DEPENDENCY_BY_REGION_BIT,
+        VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT,
+        VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT,
+        VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT,
+        VK_ACCESS_SHADER_READ_BIT);
+    }
+  }
+
+
   void DxvkContext::initImage(
     const Rc<DxvkImage>&           image,
     const VkImageSubresourceRange& subresources,

src/dxvk/dxvk_context.h

@@ -608,6 +608,14 @@ namespace dxvk {
             uint32_t          counterDivisor,
             uint32_t          counterBias);
     
+    /**
+     * \brief Emits barrier for render target readback
+     *
+     * Use between draw calls if the fragment shader
+     * reads one of the currently bound render targets.
+     */
+    void emitRenderTargetReadbackBarrier();
+
     /**
      * \brief Generates mip maps
      * 

src/dxvk/dxvk_renderpass.cpp

@@ -151,6 +151,18 @@ namespace dxvk {
         VK_ACCESS_TRANSFORM_FEEDBACK_COUNTER_READ_BIT_EXT, 0 };
     }
 
+    if (ops.barrier.srcStages & (
+          VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT |
+          VK_PIPELINE_STAGE_ALL_GRAPHICS_BIT |
+          VK_PIPELINE_STAGE_ALL_COMMANDS_BIT)) {
+      subpassDeps[subpassDepCount++] = { 0, 0,
+        VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT,
+        VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT,
+        VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT,
+        VK_ACCESS_SHADER_READ_BIT,
+        VK_DEPENDENCY_BY_REGION_BIT };
+    }
+
     if (ops.barrier.srcStages && ops.barrier.dstStages) {
       subpassDeps[subpassDepCount++] = {
         0, VK_SUBPASS_EXTERNAL,