skip SWAP_BEHAVIOR_PRESERVED for supportsBufferAge

pointless and unsupported on latter MESA/DRI3
http://lists.freedesktop.org/archives/mesa-dev/2015-November/100869.html

Thanks to Uriy Zhuravlev for reporting and investigation

BUG: 356992
CCBUG: 356882
FIXED-IN: 5.5.4
REVIEW: 126783

eglonxbackend.cpp

@@ -89,6 +89,8 @@ EglOnXBackend::~EglOnXBackend()
 
 void EglOnXBackend::init()
 {
+    initEGL();       // required to toggle
+    initBufferAge(); // EGL_SWAP_BEHAVIOR_PRESERVED_BIT
     if (!initRenderingContext()) {
         setFailed(QStringLiteral("Could not initialize rendering context"));
         return;
@@ -267,7 +269,7 @@ EGLSurface EglOnXBackend::createSurface(xcb_window_t window)
 bool EglOnXBackend::initBufferConfigs()
 {
     const EGLint config_attribs[] = {
-        EGL_SURFACE_TYPE,         EGL_WINDOW_BIT|EGL_SWAP_BEHAVIOR_PRESERVED_BIT,
+        EGL_SURFACE_TYPE,         EGL_WINDOW_BIT | (supportsBufferAge() ? 0 : EGL_SWAP_BEHAVIOR_PRESERVED_BIT),
         EGL_RED_SIZE,             1,
         EGL_GREEN_SIZE,           1,
         EGL_BLUE_SIZE,            1,

libkwineffects/kwinglutils.cpp

@@ -96,6 +96,8 @@ void initGLX()
 void initEGL()
 {
     EGLDisplay dpy = eglGetCurrentDisplay();
+    if (dpy == EGL_NO_DISPLAY)
+        dpy = eglGetDisplay(EGL_DEFAULT_DISPLAY);
     int major, minor;
     eglInitialize(dpy, &major, &minor);
     eglVersion = MAKE_GL_VERSION(major, minor, 0);