Added detection for compositors

src/slop.cpp

     // Set up x11 temporarily
     x11 = new X11(options->xdisplay);
     keyboard = new Keyboard( x11 );
-    // First we try to make an OpenGL enabled window
     bool success = false;
     SlopWindow* window;
-    try {
-        window = new SlopWindow();
-        success = true;
-    } catch (...) {
-        success = false;
+    // First we check if we have a compositor available
+	if ( x11->hasCompositor() ) {
+        // If we have a compositor, we try using OpenGL
+        try {
+            window = new SlopWindow();
+            success = true;
+        } catch (...) {
+            success = false;
+        }
+    } else {
+        std::cerr << "Failed to detect a compositor, OpenGL hardware-accelleration disabled...\n";
     }
     if ( !success ) {
         // If we fail, we launch the XShape version of slop.

src/x.cpp

 #include "x.hpp"
 
+bool X11::hasCompositor() {
+    std::stringstream prop_name;
+    prop_name << "_NET_WM_CM_S" << XScreenNumberOfScreen( screen );
+    Atom prop_atom = XInternAtom(display, prop_name.str().c_str(), False);
+    return XGetSelectionOwner(display, prop_atom) != None;
+}
+
 X11::X11( std::string displayName ) {
     // Initialize display
     display = XOpenDisplay( displayName.c_str() );

src/x.hpp

 #include <iostream>
 #include <X11/Xlib.h>
 #include <string>
+#include <sstream>
 #include <stdexcept>
+#include <X11/Xatom.h>
 
 class X11 {
 public:
+    bool hasCompositor();
     X11( std::string displayName );
     ~X11();
     Display* display;