Adds GL swap control for X11

src/Fl_Gl_Window_Driver.H

@@ -70,7 +70,7 @@ public:
   virtual void resize(int /*is_a_resize*/, int /*w*/, int /*h*/) {}
   virtual char swap_type();
   virtual void swap_interval(int) { }
-  virtual int swap_interval() { return -1; }
+  virtual int swap_interval() const { return -1; }
   virtual int flush_begin(char&) {return 0;}
   virtual void gl_hide_before(void *&) {} // the default implementation may be enough
   static Fl_Gl_Choice *find_begin(int m, const int *alistp);

src/drivers/Cocoa/Fl_Cocoa_Gl_Window_Driver.H

@@ -39,7 +39,7 @@ class Fl_Cocoa_Gl_Window_Driver : public Fl_Gl_Window_Driver {
   void resize(int is_a_resize, int w, int h) FL_OVERRIDE;
   char swap_type() FL_OVERRIDE;
   void swap_interval(int) FL_OVERRIDE;
-  int swap_interval() FL_OVERRIDE;
+  int swap_interval() const FL_OVERRIDE;
   Fl_Gl_Choice *find(int m, const int *alistp) FL_OVERRIDE;
   GLContext create_gl_context(Fl_Window* window, const Fl_Gl_Choice* g) FL_OVERRIDE;
   void set_gl_context(Fl_Window* w, GLContext context) FL_OVERRIDE;

src/drivers/Cocoa/Fl_Cocoa_Gl_Window_Driver.mm

@@ -362,7 +362,7 @@ void Fl_Cocoa_Gl_Window_Driver::swap_interval(int n) {
     [ctx setValues:&interval forParameter:NSOpenGLContextParameterSwapInterval];
 }
 
-int Fl_Cocoa_Gl_Window_Driver::swap_interval() {
+int Fl_Cocoa_Gl_Window_Driver::swap_interval() const {
   GLint interval = (GLint)-1;
   NSOpenGLContext* ctx = (NSOpenGLContext*)pWindow->context();
   if (ctx)

src/drivers/X11/Fl_X11_Gl_Window_Driver.H

@@ -32,6 +32,8 @@ class Fl_X11_Gl_Window_Driver : public Fl_Gl_Window_Driver {
   int mode_(int m, const int *a) FL_OVERRIDE;
   void swap_buffers() FL_OVERRIDE;
   char swap_type() FL_OVERRIDE;
+  void swap_interval(int) FL_OVERRIDE;
+  int swap_interval() const FL_OVERRIDE;
   Fl_Gl_Choice *find(int m, const int *alistp) FL_OVERRIDE;
   GLContext create_gl_context(Fl_Window* window, const Fl_Gl_Choice* g) FL_OVERRIDE;
   void set_gl_context(Fl_Window* w, GLContext context) FL_OVERRIDE;

src/drivers/X11/Fl_X11_Gl_Window_Driver.cxx

@@ -393,14 +393,49 @@ void Fl_X11_Gl_Window_Driver::swap_buffers() {
   glXSwapBuffers(fl_display, fl_xid(pWindow));
 }
 
+char Fl_X11_Gl_Window_Driver::swap_type() {
+  return copy;
+}
 
-char Fl_X11_Gl_Window_Driver::swap_type() {return copy;}
+typedef void (*SWAPINTERVALPROC) (Display *dpy, GLXDrawable drawable, int interval);
+static SWAPINTERVALPROC glXSwapIntervalEXT = NULL;
+static bool glXSwapIntervalChecked = false;
+static void checkGlxXSwapInterval() {
+  if (!glXSwapIntervalChecked) {
+    glXSwapIntervalChecked = true;
+    glXSwapIntervalEXT = (SWAPINTERVALPROC)glXGetProcAddressARB((const GLubyte*)"glXSwapIntervalEXT");
+  }
+}
+
+void Fl_X11_Gl_Window_Driver::swap_interval(int interval) {
+  if (!fl_xid(pWindow))
+    return;
+  if (!glXSwapIntervalChecked) checkGlxXSwapInterval();
+  if (glXSwapIntervalEXT) {
+    glXSwapIntervalEXT(fl_display, fl_xid(pWindow), interval);
+  }
+}
+
+int Fl_X11_Gl_Window_Driver::swap_interval() const {
+  if (!fl_xid(pWindow))
+    return -1;
+  if (!glXSwapIntervalChecked) checkGlxXSwapInterval();
+  if (!glXSwapIntervalEXT) return -1;
+  static bool ext_checked = false, ext_exists = false;
+  if (!ext_checked) {
+    ext_checked = true;
+    ext_exists = (strstr(glXQueryExtensionsString(fl_display, fl_screen), "GLX_EXT_swap_control") != NULL);
+  }
+  if (!ext_exists) return -1;
+  unsigned int interval = -1;
+  glXQueryDrawable(fl_display, fl_xid(pWindow), 0x20F1 /*GLX_SWAP_INTERVAL_EXT*/, &interval);
+  return interval;
+}
 
 void Fl_X11_Gl_Window_Driver::waitGL() {
   glXWaitGL();
 }
 
-
 void Fl_X11_Gl_Window_Driver::gl_visual(Fl_Gl_Choice *c) {
   Fl_Gl_Window_Driver::gl_visual(c);
   fl_visual = ((Fl_X11_Gl_Choice*)c)->vis;