Lazily create readback buffer

* This means we don't allocate a large buffer even though we won't actually use
  it. Most important for memory constrained platforms like android.

renderdoc/driver/vulkan/vk_debug.cpp

@@ -2204,6 +2204,19 @@ void VulkanDebugManager::FillWithDiscardPattern(VkCommandBuffer cmd, DiscardType
   DoPipelineBarrier(cmd, 1, &dstimBarrier);
 }
 
+void VulkanDebugManager::InitReadbackBuffer()
+{
+  if(m_ReadbackWindow.buf == VK_NULL_HANDLE)
+  {
+    VkDevice dev = m_pDriver->GetDev();
+    m_ReadbackWindow.Create(m_pDriver, dev, 256 * 1024 * 1024ULL, 1, GPUBuffer::eGPUBufferReadback);
+
+    VkResult vkr = ObjDisp(dev)->MapMemory(Unwrap(dev), Unwrap(m_ReadbackWindow.mem), 0,
+                                           VK_WHOLE_SIZE, 0, (void **)&m_ReadbackPtr);
+    RDCASSERTEQUAL(vkr, VK_SUCCESS);
+  }
+}
+
 void VulkanReplay::PatchReservedDescriptors(const VulkanStatePipeline &pipe,
                                             VkDescriptorPool &descpool,
                                             rdcarray<VkDescriptorSetLayout> &setLayouts,

renderdoc/driver/vulkan/vk_debug.h

@@ -72,6 +72,7 @@ public:
                               VkImageLayout curLayout, VkImageSubresourceRange discardRange,
                               VkRect2D discardRect);
 
+  void InitReadbackBuffer();
   byte *GetReadbackPtr() { return m_ReadbackPtr; }
   VkBuffer GetReadbackBuffer() { return m_ReadbackWindow.buf; }
   VkDeviceMemory GetReadbackMemory() { return m_ReadbackWindow.mem; }

renderdoc/driver/vulkan/wrappers/vk_queue_funcs.cpp

@@ -1077,6 +1077,8 @@ VkResult WrappedVulkan::vkQueueSubmit(VkQueue queue, uint32_t submitCount,
             RDCDEBUG("Reading back %s with GPU for comparison",
                      ToStr(record->GetResourceID()).c_str());
 
+            GetDebugManager()->InitReadbackBuffer();
+
             // immediately issue a command buffer to copy back the data. We do that on this queue to
             // avoid complexity with synchronising with another queue, but the transfer queue if
             // available would be better for this purpose.