Bug fix: GLX backend incompatibility with mesa & others

- Fix a bug that glx_bind_pixmap() doesn't work with mesa drivers.
  Thanks to Janhouse and mkraemer for reporting. (#7)

- Use stencil buffer to attempt to eliminate potential double-paint
  issue in glx_render(). X Fixes doesn't guarantee the rectangles in a
  region do not overlap, and this may cause some regions to be painted
  twice, which would be a problem if we are painting transparent things.
  Now the target window must have a stencil buffer. Compiz uses its own
  region implementation to deal with this, but as a lightweight
  compositor we can't really do the same. It may have a positive or
  negative effort over performance. Callgrind result indicates basically
  no change in performance, but this may or may not be true.

- Correctly distinguish GL extensions and GLX extensions. Sorry. :-D

- Handle screen size. Thanks to tsmithe for reporting. (#7)

- Rename OpenGL backend to GLX backend, because, we might have a EGL
  backend someday.

- Add configuration file option `backend` to specify backend. Add
  `backend` to D-Bus `opts_get`.

- Add OpenGL shader compilation code, but currently unused.

- Minor adjustments.

- Known issue: Window content doesn't get updated in VirtualBox,
  probably because its OpenGL implementation requires constant rebinding
  of texture. But that's really slow...

- Known issue: Blur feature is still unimplemented in GLX backend.

src/opengl.h

@@ -12,22 +12,62 @@
 
 #include <ctype.h>
 
+/**
+ * Check if a word is in string.
+ */
+static inline bool
+wd_is_in_str(const char *haystick, const char *needle) {
+  if (!haystick)
+    return false;
+
+  assert(*needle);
+
+  const char *pos = haystick - 1;
+  while ((pos = strstr(pos + 1, needle))) {
+    // Continue if it isn't a word boundary
+    if (((pos - haystick) && !isspace(*(pos - 1)))
+        || (strlen(pos) > strlen(needle) && !isspace(pos[strlen(needle)])))
+      continue;
+    return true;
+  }
+
+  return false;
+}
+
 /**
  * Check if a GLX extension exists.
  */
 static inline bool
-glx_hasext(session_t *ps, const char *ext) {
+glx_hasglxext(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);
+  if (!glx_exts) {
+    printf_errf("(): Failed get GLX extension list.");
     return false;
   }
 
-  return true;
+  bool found = wd_is_in_str(glx_exts, ext);
+  if (!found)
+    printf_errf("(): Missing GLX extension %s.", ext);
+
+  return found;
+}
+
+/**
+ * Check if a GLX extension exists.
+ */
+static inline bool
+glx_hasglext(session_t *ps, const char *ext) {
+  const char *gl_exts = (const char *) glGetString(GL_EXTENSIONS);
+  if (!gl_exts) {
+    printf_errf("(): Failed get GL extension list.");
+    return false;
+  }
+
+  bool found = wd_is_in_str(gl_exts, ext);
+  if (!found)
+    printf_errf("(): Missing GL extension %s.", ext);
+
+  return found;
 }
 
 static inline XVisualInfo *