Feature: OpenGL backend

- Add experimental OpenGL backend (--opengl). --blur-background is
  currently not possible with this backend, because I'm still trying to
  find a proper way to do blur with OpenGL. Flipping backend on-the-fly
  is really hard, so it isn't supported right now. No configuration file
  option exists to enable this, because it isn't stable enough.

- Add `opengl-swc` VSync method that uses SGI_swap_control to control
  buffer swap, with OpenGL backend. (#7)

- Fix a potential read-from-freed-memory issue in paint_all().

- Correctly reattach GLX context after fork.

- Dump error text in error(). Add GLX error code handling.

- Code clean-up.

- Known issues: Region operations take a lot of time in glx_render().
  I'm hesitating about what to do.

src/opengl.h

@@ -0,0 +1,46 @@
+/*
+ * Compton - a compositor for X11
+ *
+ * Based on `xcompmgr` - Copyright (c) 2003, Keith Packard
+ *
+ * Copyright (c) 2011-2013, Christopher Jeffrey
+ * See LICENSE for more information.
+ *
+ */
+
+#include "common.h"
+
+#include <ctype.h>
+
+/**
+ * Check if a GLX extension exists.
+ */
+static inline bool
+glx_hasext(session_t *ps, const char *ext) {
+  const char *glx_exts = glXQueryExtensionsString(ps->dpy, ps->scr);
+  const char *pos = strstr(glx_exts, ext);
+  // Make sure the extension string is matched as a whole word
+  if (!pos
+      || ((pos - glx_exts) && !isspace(*(pos - 1)))
+      || (strlen(pos) > strlen(ext) && !isspace(pos[strlen(ext)]))) {
+    printf_errf("(): Missing OpenGL extension %s.", ext);
+    return false;
+  }
+
+  return true;
+}
+
+static inline XVisualInfo *
+get_visualinfo_from_visual(session_t *ps, Visual *visual) {
+  XVisualInfo vreq = { .visualid = XVisualIDFromVisual(visual) };
+  int nitems = 0;
+
+  return XGetVisualInfo(ps->dpy, VisualIDMask, &vreq, &nitems);
+}
+
+static bool
+glx_update_fbconfig(session_t *ps);
+
+static int
+glx_cmp_fbconfig(session_t *ps,
+    const glx_fbconfig_t *pfbc_a, const glx_fbconfig_t *pfbc_b);