Bug fix #91: Using pkg-config to find drm.h & OpenGL changes

- #91: Use pkg-config to find drm.h to avoid issues on FreeBSD. Thanks
  to hun7err for pointing out and providing patch.

- #89: Add default shadow exclusion rule for notify-osd. Thanks to
  DanielRS.

- Check for abundant positional arguments.

- Use paint target window (root window / overlay window) instead of
  ps->reg_win to create GLXContext. (May have negative effects on OpenGL
  VSync.) Add new OpenGL helpers functions, to prepare for the new
  OpenGL backend.

- Dump more info of a PropertyNotify with DEBUG_EVENTS.

src/common.h

@@ -258,6 +258,14 @@ typedef int (*f_GetVideoSync) (unsigned *);
 
 typedef Bool (*f_GetSyncValuesOML) (Display* dpy, GLXDrawable drawable, int64_t* ust, int64_t* msc, int64_t* sbc);
 typedef Bool (*f_WaitForMscOML) (Display* dpy, GLXDrawable drawable, int64_t target_msc, int64_t divisor, int64_t remainder, int64_t* ust, int64_t* msc, int64_t* sbc);
+
+typedef void (*f_BindTexImageEXT) (Display *display, GLXDrawable drawable, int buffer, const int *attrib_list);
+typedef void (*f_ReleaseTexImageEXT) (Display *display, GLXDrawable drawable, int buffer);
+
+struct glx_fbconfig {
+  GLXFBConfig cfg;
+  bool y_inverted;
+};
 #endif
 
 typedef struct {
@@ -536,7 +544,7 @@ typedef struct {
 #endif
 
 #ifdef CONFIG_VSYNC_OPENGL
-  // === OpenGL VSync related ===
+  // === OpenGL related ===
   /// GLX context.
   GLXContext glx_context;
   /// Pointer to glXGetVideoSyncSGI function.
@@ -547,6 +555,14 @@ typedef struct {
   f_GetSyncValuesOML glXGetSyncValuesOML;
   /// Pointer to glXWaitForMscOML function.
   f_WaitForMscOML glXWaitForMscOML;
+  /// Pointer to glXBindTexImageEXT function.
+  f_BindTexImageEXT glXBindTexImageEXT;
+  /// Pointer to glXReleaseTexImageEXT function.
+  f_ReleaseTexImageEXT glXReleaseTexImageEXT;
+  /// FBConfig for RGB GLX pixmap.
+  struct glx_fbconfig *glx_fbconfig_rgb;
+  /// FBConfig for RGBA GLX pixmap.
+  struct glx_fbconfig *glx_fbconfig_rgba;
 #endif
 
   // === X extension related ===