First step of split backend into modules

This commit introduced a new, modular backend interface. The interface
is not very good, since I don't think I fully understand all the
requirements writing a backend have. But this is a good first step.

This commit also includes an initial xrender backend written using the
new interface, and some opengl backend related helper functions, which
are taken from the old opengl backend.

However, there is not integration with the core compton yet. compton
will still use the old backend code. This commit is here so we can get
the automated build test.

What is implemented in the new xrender backend:

* Windows with transparency
* Shadow
* Opacity
* Wallpaper (getting the root pixmap)
* Blur

Known problem with the xrender backend:

* It is slower

Things that still need to be figured out:

* What is the better way to add vsync to the new backends

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/win.h

@@ -9,7 +9,6 @@
 
 // FIXME shouldn't need this
 #ifdef CONFIG_OPENGL
-#define GL_GLEXT_PROTOTYPES
 #include <GL/glx.h>
 #endif
 
@@ -75,6 +74,9 @@ typedef enum {
 /// Structure representing a top-level window compton manages.
 typedef struct win win;
 struct win {
+  /// backend data attached to this window. Only available when
+  /// `state` is not UNMAPPED
+  void *win_data;
   /// Pointer to the next lower window in window stack.
   win *next;
   /// Pointer to the next higher window to paint.