Bug fix: GLX: ARGB texture too dark & Jitter when resize & others

- GLX backend: Fix a bug that ARGB windows / shadows are rendered too
  dark. Thanks to derhass in FreeNode/##opengl for help.

- GLX backend: Fix a problem that during window resize the content looks
  jittering, by letting compton fetch pixmap sizes with XGetGeometry()
  instead of relying on window width/height, which could be inaccurate
  during window resize. Negative effect on performance. Thanks to M4he
  for reporting. (#7)

- Add .desktop file. Thanks to quequotion for providing it. (#97)

- Avoid checking presence of window pixmap, because they may not exist
  with very old X Composite implementations.

- Add workaround for a strange window restack issue when compton
  receieves a ConfigureNotify with non-existent new above window.

- Add debugging function hexdump(). Extra sanity checks on various
  places.

compton.desktop

@@ -0,0 +1,10 @@
+[Desktop Entry]
+Version=1.0
+Type=Application
+Name=compton
+Comment=A X compositor
+Categories=Utility;
+TryExec=compton
+Exec=compton
+Icon=xcompmgr
+# Thanks to quequotion for providing this file!