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/dbus.c

@@ -876,6 +876,11 @@ cdbus_process_opts_get(session_t *ps, DBusMessage *msg) {
     cdbus_reply_string(ps, msg, VSYNC_STRS[ps->o.vsync]);
     return true;
   }
+  if (!strcmp("backend", target)) {
+    assert(ps->o.backend < sizeof(BACKEND_STRS) / sizeof(BACKEND_STRS[0]));
+    cdbus_reply_string(ps, msg, BACKEND_STRS[ps->o.backend]);
+    return true;
+  }
   cdbus_m_opts_get_do(dbe, cdbus_reply_bool);
   cdbus_m_opts_get_do(vsync_aggressive, cdbus_reply_bool);