Fixed all EGLConfigs listed as single-buffered.

The commit c980858 added a new flag to _GLFWfbconfig that was not being
set by EGLConfig enumeration.  This patch assumes eglCreateWindowSurface
only creates double-buffered surfaces.

Fixes #315.

src/egl_context.c

@@ -167,6 +167,7 @@ static GLboolean chooseFBConfigs(const _GLFWctxconfig* ctxconfig,
         u->stencilBits = getConfigAttrib(n, EGL_STENCIL_SIZE);
 
         u->samples = getConfigAttrib(n, EGL_SAMPLES);
+        u->doublebuffer = GL_TRUE;
 
         u->egl = n;
         usableCount++;