When first rendering to output window backbuffer, clear it.

* This fixes a validation error which complains (rightly) that our
  renderpass writing to the backbuffer is LOAD_OP_LOAD but we haven't
  initialised the memory. So just clear to black - the contents don't
  matter as we'll just be overwriting it entirely.

renderdoc/driver/vulkan/vk_replay.cpp

@@ -44,6 +44,8 @@ VulkanReplay::OutputWindow::OutputWindow() : wnd(NULL_WND_HANDLE), width(0), hei
 	for(size_t i=0; i < ARRAY_COUNT(colimg); i++)
 		colimg[i] = VK_NULL_HANDLE;
 
+	fresh = true;
+
 	bb = VK_NULL_HANDLE;
 	bbview = VK_NULL_HANDLE;
 	fb = VK_NULL_HANDLE;
@@ -168,6 +170,8 @@ void VulkanReplay::OutputWindow::Create(WrappedVulkan *driver, VkDevice device,
 
 	surface = oldsurf;
 
+	fresh = true;
+
 	if(surface == VK_NULL_HANDLE)
 	{
 		CreateSurface(inst);
@@ -2565,6 +2569,23 @@ void VulkanReplay::BindOutputWindow(uint64_t id, bool depth)
 
 	outw.depthBarrier.newLayout = VK_IMAGE_LAYOUT_DEPTH_STENCIL_ATTACHMENT_OPTIMAL;
 	
+	// first time rendering to the backbuffer, clear it, since our typical render pass
+	// is set to LOAD_OP_LOAD
+	if(outw.fresh)
+	{
+		outw.bbBarrier.newLayout = VK_IMAGE_LAYOUT_TRANSFER_DST_OPTIMAL;
+		outw.bbBarrier.dstAccessMask = VK_ACCESS_TRANSFER_WRITE_BIT;
+
+		DoPipelineBarrier(cmd, 1, &outw.bbBarrier);
+		float black[] = { 0.0f, 0.0f, 0.0f, 0.0f };
+		vt->CmdClearColorImage(Unwrap(cmd), Unwrap(outw.bb), VK_IMAGE_LAYOUT_TRANSFER_DST_OPTIMAL, (VkClearColorValue *)black, 1, &outw.bbBarrier.subresourceRange);
+
+		outw.bbBarrier.oldLayout = outw.bbBarrier.newLayout;
+		outw.bbBarrier.srcAccessMask = outw.bbBarrier.dstAccessMask;
+
+		outw.fresh = false;
+	}
+	
 	outw.bbBarrier.newLayout = VK_IMAGE_LAYOUT_COLOR_ATTACHMENT_OPTIMAL;
 	outw.bbBarrier.dstAccessMask = VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT;
 	outw.colBarrier[outw.curidx].newLayout = VK_IMAGE_LAYOUT_TRANSFER_DST_OPTIMAL;

renderdoc/driver/vulkan/vk_replay.h

@@ -193,6 +193,8 @@ class VulkanReplay : public IReplayDriver
 
 			WINDOW_HANDLE_DECL
 
+			bool fresh;
+
 			uint32_t width, height;
 
 			VkSurfaceKHR surface;