Offset position in X11 for east and south gravity

Adds new virtual functions x_from_right() and y_from_bottom() to
Fl_Screen_Driver.

These are overridden in Fl_X11_Screen_Driver to return the offset
between the actual screen edge (right or bottom) and the scaled up
screen edge.  The calculation is:

scaled delta = scaled limit - scaled coordinate
real coordinate = real limit - (scaled delta * screen scale)

These offsets then applied when creating a window with east or south
gravity to adjust for the loss of precision in the screen dimensions
when they were scaled.

src/Fl_Screen_Driver.H

@@ -111,6 +111,8 @@ public:
   virtual int y() { return 0; }
   virtual int w() { return 800; } // default, FL_OVERRIDE in driver!
   virtual int h() { return 600; } // default, FL_OVERRIDE in driver!
+  virtual int x_from_right(int x) { return x * scale(0); }
+  virtual int y_from_bottom(int y) { return y * scale(0); }
   virtual int screen_count();
   void screen_xywh(int &X, int &Y, int &W, int &H, int mx, int my);
   virtual void screen_xywh(int &X, int &Y, int &W, int &H, int /*n*/) {

src/Fl_x.cxx

@@ -2487,10 +2487,34 @@ void Fl_X::make_xid(Fl_Window* win, XVisualInfo *visual, Colormap colormap)
   s = Fl::screen_driver()->scale(nscreen);
   // if (!win->parent()) printf("win creation on screen #%d\n", nscreen);
 #endif
+
+  int win_x = rint(X*s);
+  int win_y = rint(Y*s);
+
+  switch (Fl_X11_Window_Driver::driver(win)->win_gravity_) {
+    case Fl_X11_Window_Driver::WIN_GRAVITY_NORTHEAST:
+    case Fl_X11_Window_Driver::WIN_GRAVITY_EAST:
+    case Fl_X11_Window_Driver::WIN_GRAVITY_SOUTHEAST:
+      win_x = Fl::screen_driver()->x_from_right(X);
+      break;
+    default:
+      break;
+  }
+
+  switch (Fl_X11_Window_Driver::driver(win)->win_gravity_) {
+    case Fl_X11_Window_Driver::WIN_GRAVITY_SOUTHWEST:
+    case Fl_X11_Window_Driver::WIN_GRAVITY_SOUTH:
+    case Fl_X11_Window_Driver::WIN_GRAVITY_SOUTHEAST:
+      win_y = Fl::screen_driver()->y_from_bottom(Y);
+      break;
+    default:
+      break;
+  }
+
   Fl_X* xp =
     set_xid(win, XCreateWindow(fl_display,
                                root,
-                               rint(X*s), rint(Y*s), W*s, H*s,
+                               win_x, win_y, W*s, H*s,
                                0, // borderwidth
                                visual->depth,
                                InputOutput,

src/drivers/X11/Fl_X11_Screen_Driver.H

@@ -81,6 +81,8 @@ public:
   int y() FL_OVERRIDE;
   int w() FL_OVERRIDE;
   int h() FL_OVERRIDE;
+  int x_from_right(int x) FL_OVERRIDE;
+  int y_from_bottom(int y) FL_OVERRIDE;
   void screen_xywh(int &X, int &Y, int &W, int &H, int n) FL_OVERRIDE;
   void screen_dpi(float &h, float &v, int n=0) FL_OVERRIDE;
   void screen_work_area(int &X, int &Y, int &W, int &H, int n) FL_OVERRIDE;

src/drivers/X11/Fl_X11_Screen_Driver.cxx

@@ -26,6 +26,7 @@
 #include <FL/Fl_Image_Surface.H>
 #include <FL/Fl_Tooltip.H>
 #include <FL/filename.H>
+#include <math.h>
 #include <sys/time.h>
 
 #include "../../Fl_Timeout.h"
@@ -252,6 +253,22 @@ int Fl_X11_Screen_Driver::h() {
   ;
 }
 
+// Returns a real (unscaled) X coordinate calculated relative to the right of
+// of the screen.  Used when positioning window with "east" gravity.
+
+int Fl_X11_Screen_Driver::x_from_right(int x) {
+	int dx = w() - x;
+	return (fl_workarea_xywh[0] + fl_workarea_xywh[2]) - rint(dx * screens[0].scale);
+}
+
+// Returns a real (unscaled) Y coordinate calculated relative to the bottom of
+// the screen.  Used when positioning window with "south" gravity.
+
+int Fl_X11_Screen_Driver::y_from_bottom(int y) {
+	int dy = h() - y;
+	return (fl_workarea_xywh[1] + fl_workarea_xywh[3]) - rint(dy * screens[0].scale);
+}
+
 #define USE_XRANDR (HAVE_DLSYM && HAVE_DLFCN_H) // means attempt to dynamically load libXrandr.so
 #if USE_XRANDR
 #include <dlfcn.h>