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/compton.h

@@ -176,7 +176,7 @@ paint_isvalid(session_t *ps, const paint_t *ppaint) {
   return true;
 }
 /**
- * Bind texture in paint_t if we are using OpenGL backend.
+ * Bind texture in paint_t if we are using GLX backend.
  */
 static inline bool
 paint_bind_tex(session_t *ps, paint_t *ppaint, int wid, int hei, int depth) {
@@ -542,6 +542,20 @@ win_render(session_t *ps, win *w, int x, int y, int wid, int hei, double opacity
       pict, (w ? w->paint.ptex: ps->root_tile_paint.ptex), reg_paint);
 }
 
+static inline void
+set_tgt_clip(session_t *ps, XserverRegion reg) {
+  switch (ps->o.backend) {
+    case BKEND_XRENDER:
+      XFixesSetPictureClipRegion(ps->dpy, ps->tgt_buffer, 0, 0, reg);
+      break;
+#ifdef CONFIG_VSYNC_OPENGL
+    case BKEND_GLX:
+      glx_set_clip(ps, reg);
+      break;
+#endif
+  }
+}
+
 static void
 paint_all(session_t *ps, XserverRegion region, win *t);