Imp: Multi-pass blur & D-Bus fading control

- Add multipass blur support. Note GLX Framebuffer support is required.
  My benchmark shows multipass blur brings 5% performance boost for X
  Render backend (3x3box). On GLX backend it brings 10% performance
  boost for 5x5box but negatively affects performance for 3x3box. Thanks
  to jrfonseca for advice. (#107)

- GLX backend: Cache blur texture for each window, for a 12% performance
  boost.

- Add D-Bus fading control. Thanks to yulan6248 for testing. (#112)

- Fix FAQ link in README.md. Thanks to lorenzos for report. (#111)

- Correctly deinitialize VSync on on-the-fly VSync method switch.

- X Render backend: Normalize blur kernel.

- Code clean-up.

- Known issue: Linear corruption on border of a window may appear with X
  Render multi-pass blur. Possible to fix but probably not worthwhile.

Makefile

@@ -52,6 +52,12 @@ ifeq "$(NO_VSYNC_OPENGL)" ""
   ifeq "$(NO_VSYNC_OPENGL_GLSL)" ""
     CFG += -DCONFIG_VSYNC_OPENGL_GLSL
   endif
+  ifeq "$(NO_VSYNC_OPENGL_FBO)" ""
+    CFG += -DCONFIG_VSYNC_OPENGL_FBO
+  endif
+  ifeq "$(NO_VSYNC_OPENGL_VBO)" ""
+    CFG += -DCONFIG_VSYNC_OPENGL_VBO
+  endif
 endif
 
 # ==== D-Bus ====
@@ -73,7 +79,7 @@ CFG += -DCOMPTON_VERSION="\"$(COMPTON_VERSION)\""
 
 LDFLAGS ?= -Wl,-O1 -Wl,--as-needed
 
-ifeq "$(DEV)" ""
+ifeq "$(CFG_DEV)" ""
   CFLAGS ?= -DNDEBUG -O2 -D_FORTIFY_SOURCE=2
 else
   CC = clang