compositor: make XPresent VSync default

Compared with GLX, XPresent is more performant,
proven in testing with:
nouveau, radeon and intel - can be tested with:

$ LIBGL_DEBUG=verbose vblank_mode=0 glxgears

using the following configuration:

/etc/X11/xorg.conf.d/nouveau-dri3.conf
Section "Device"
	Identifier  "nvidia0"
	Driver      "nouveau"
	Option      "DRI" "3"
EndSection

Xorg.0.log:
NOUVEAU(0): DRI3 on EXA enabled
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/etc/X11/xorg.conf.d/radeon-dri3.conf
Section "Device"
	Identifier  "amd0"
	Driver      "radeon"
	Option      "DRI3" "on"
EndSection

Xorg.0.log:
RADEON(0): DRI3 enabled
~~~~~~~~~~~~~~~~~~~~~~~

/etc/X11/xorg.conf.d/intel-dri3.conf
Section "Device"
	Identifier  "intel0"
	Driver      "intel"
	Option      "DRI" "3"
EndSection

Xorg.0.log:
intel(0): direct rendering: DRI2 DRI3 enabled
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In some cases, the user may prefer GLX over XPresent VSync,
if both are present - can be checked with:

$ xfwm4 --version

An environment variable XFWM4_USE_PRESENT is for this purpose:

$ XFWM4_USE_PRESENT=0 xfwm4 --replace &

this will select GLX VSync even if XPresent is available.

---
 src/compositor.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/compositor.c b/src/compositor.c
index dc45218..1bbae53 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -4351,14 +4351,14 @@ compositorManageScreen (ScreenInfo *screen_info)
     screen_info->texture_filter = GL_LINEAR;
 #ifdef HAVE_PRESENT_EXTENSION
     use_present_env = g_getenv ("XFWM4_USE_PRESENT");
-    if (g_strcmp0 (use_present_env, "1") == 0)
+    if (g_strcmp0 (use_present_env, "0") == 0)
     {
-        screen_info->use_glx = FALSE;
+        screen_info->use_glx = init_glx (screen_info);
     }
     else
 #endif /* HAVE_PRESENT_EXTENSION */
     {
-        screen_info->use_glx = init_glx (screen_info);
+        screen_info->use_glx = FALSE;
     }
 #else /* HAVE_EPOXY */
     screen_info->use_glx = FALSE;
-- 
2.6.3