keep buffer swap pref automatic until GPU detected

enforce to "e" (cheap) when driver is still unknown after
detection must be assumed to have run, so a sane value is available
when the context is up

BUG: 322355
FIXED-IN: 4.11
REVIEW: 111548

eglonxbackend.cpp

@@ -74,6 +74,9 @@ void EglOnXBackend::init()
     glPlatform->detect(EglPlatformInterface);
     if (GLPlatform::instance()->driver() == Driver_Intel)
         options->setUnredirectFullscreen(false); // bug #252817
+    options->setGlPreferBufferSwap(options->glPreferBufferSwap()); // resolve autosetting
+    if (options->glPreferBufferSwap() == Options::AutoSwapStrategy)
+        options->setGlPreferBufferSwap('e'); // for unknown drivers - should not happen
     glPlatform->printResults();
     initGL(EglPlatformInterface);
     if (!hasGLExtension("GL_OES_EGL_image")) {

glxbackend.cpp

@@ -97,6 +97,9 @@ void GlxBackend::init()
     glPlatform->detect(GlxPlatformInterface);
     if (GLPlatform::instance()->driver() == Driver_Intel)
         options->setUnredirectFullscreen(false); // bug #252817
+    options->setGlPreferBufferSwap(options->glPreferBufferSwap()); // resolve autosetting
+    if (options->glPreferBufferSwap() == Options::AutoSwapStrategy)
+        options->setGlPreferBufferSwap('e'); // for unknown drivers - should not happen
     glPlatform->printResults();
     initGL(GlxPlatformInterface);
     // Check whether certain features are supported

options.cpp

@@ -767,7 +767,7 @@ void Options::setGlPreferBufferSwap(char glPreferBufferSwap)
         // see http://www.x.org/releases/X11R7.7/doc/dri2proto/dri2proto.txt, item 2.5
         if (GLPlatform::instance()->driver() == Driver_NVidia)
             glPreferBufferSwap = CopyFrontBuffer;
-        else
+        else if (GLPlatform::instance()->driver() != Driver_Unknown) // undetected, finally resolved when context is initialized
             glPreferBufferSwap = ExtendDamage;
     }
     if (m_glPreferBufferSwap == (GlSwapStrategy)glPreferBufferSwap) {