Feature: Add XRender-GLX hybird backend

- Add new backend "xr_glx_hybird", which uses X Render for all
  compositing but GLX on the last step of rendering to screen.  This
  makes GLX-backend-specific VSync methods usable while may avoid
  certain bugs with GLX backend. The idea comes from ali1234.
  Experimental.

- GLX backend: Stop using or rendering to depth buffer.

- Use glFinish() instead of glFlush() before VSync. It probably uses
  more CPU but could be more reliable than glFlush().

src/opengl.c

@@ -124,7 +124,8 @@ glx_init(session_t *ps, bool need_render) {
   if (need_render) {
     glx_on_root_change(ps);
 
-    // glEnable(GL_DEPTH_TEST);
+    glDisable(GL_DEPTH_TEST);
+    glDepthMask(GL_FALSE);
     glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
     glDisable(GL_BLEND);
 
@@ -880,7 +881,7 @@ glx_set_clip(session_t *ps, XserverRegion reg, const reg_data_t *pcache_reg) {
 
     glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);
     glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
-    glDepthMask(GL_TRUE);
+    // glDepthMask(GL_TRUE);
   }
 
   cxfree(rects_free);