Avoid picking worse formats when matching compatible formats

Like we do with fully matching format, pick the first matching one,
instead of the last matching one.

Fixes: QTBUG-72785
Change-Id: I466e0152a229348b6a3786d5464d1f8ab325d67a
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>

src/platformsupport/glxconvenience/qglxconvenience.cpp

@@ -241,6 +241,8 @@ GLXFBConfig qglx_findConfig(Display *display, int screen , QSurfaceFormat format
             if (requestedAlpha && actualAlpha < requestedAlpha)
                 continue;
             compatibleCandidate = candidate;
+            if (!compatibleCandidate) // Only pick up the first compatible one offered by the server
+                compatibleCandidate = candidate;
 
             if (requestedRed && actualRed != requestedRed)
                 continue;