Fix crash in XRenderPictureData::~XRenderPictureData

Summary:
If the XRender scene has cross-faded a window, then, eventually, KWin/X11 will
crash in the destructor of the XRenderPictureData class during tear down with
the following message in the terminal

```
ASSERT: "qApp" in file /home/vlad/Workspace/KDE/src/kde/workspace/kwin/libkwineffects/kwinxrenderutils.cpp, line 163
```

The crash happens because X11StandalonePlatform attempts to clean up XRender
resources, including XRenderUtils::s_blendPicture, after the application object
has been destroyed.

In order to fix the crash, we have to destroy the platform object before the
destructor of QCoreApplication is executed.

Test Plan:
- Enable maximize effect
- Maximize a window
- Replace the current instance of KWin/X11 with another one

Without this patch, KWin/X11 crashes after the third step.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25768

main.cpp

@@ -161,6 +161,7 @@ Application::~Application()
 {
     delete options;
     destroyAtoms();
+    destroyPlatform();
 }
 
 void Application::destroyAtoms()
@@ -169,6 +170,12 @@ void Application::destroyAtoms()
     atoms = nullptr;
 }
 
+void Application::destroyPlatform()
+{
+    delete m_platform;
+    m_platform = nullptr;
+}
+
 void Application::resetCrashesCount()
 {
     crashes = 0;

main.h

@@ -231,6 +231,7 @@ protected:
         emit x11ConnectionChanged();
     }
     void destroyAtoms();
+    void destroyPlatform();
 
     void setTerminating() {
         m_terminating = true;