kms: Also initialize in resize(), if required.

It appears to have been an incorrect assumption that flush always comes before
resize. At least for me, that is not the case. It also strikes me as being more
robust this way.

Change-Id: Iafd2cfe7b89802899e844152e0901e588ff0d8a6
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/plugins/platforms/kms/qkmsbackingstore.cpp

@@ -203,6 +203,11 @@ void QKmsBackingStore::resize(const QSize &size, const QRegion &staticContents)
 
     m_context->makeCurrent(window());
 
+    if (!m_initialized) {
+        initializeOpenGLFunctions();
+        m_initialized = true;
+    }
+
     if (m_texture)
         glDeleteTextures(1, &m_texture);