Vulkan: release all resources on RendererVk destroy

The most important part of this is unloading the libvulkan handle. We
open the handle in ::initialize but we were previously only unloading it
in ::~RendererVk[1]. Since a DisplayVk (and RendererVk) are not
destructed in between context creations, this meant that the libvulkan
library handle reference count only increased, never decreased.

[1] Which, incidentally, never gets invoked, ever. We create Display
    instances and they live forever in a static structure and we only
    ::initialize/::terminate them.

Bug: angleproject:8225
Change-Id: I03da9b19f0c4ae3b5efacd07880aadb7ee77ebf5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4636882
Reviewed-by: Charlie Lao <cclao@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>

src/libANGLE/renderer/vulkan/RendererVk.cpp

@@ -1412,17 +1412,7 @@ RendererVk::RendererVk()
     ASSERT(IsLittleEndian());
 }
 
-RendererVk::~RendererVk()
-{
-    mAllocator.release();
-    mPipelineCache.release();
-    ASSERT(!hasSharedGarbage());
-
-    if (mLibVulkanLibrary)
-    {
-        angle::CloseSystemLibrary(mLibVulkanLibrary);
-    }
-}
+RendererVk::~RendererVk() {}
 
 bool RendererVk::hasSharedGarbage()
 {
@@ -1496,6 +1486,14 @@ void RendererVk::onDestroy(vk::Context *context)
 
     mEnabledInstanceExtensions.clear();
     mEnabledDeviceExtensions.clear();
+
+    ASSERT(!hasSharedGarbage());
+
+    if (mLibVulkanLibrary)
+    {
+        angle::CloseSystemLibrary(mLibVulkanLibrary);
+        mLibVulkanLibrary = nullptr;
+    }
 }
 
 void RendererVk::notifyDeviceLost()