Fix for issue #159 - continued

Changes here concentrate the fix within static function alpha_blend() and thus
are visible only when drawing transparent images under X11 platform
without Xrender.

src/drivers/Xlib/Fl_Xlib_Graphics_Driver_image.cxx

@@ -658,13 +658,15 @@ static void alpha_blend(Fl_RGB_Image *img, int X, int Y, int W, int H, int cx, i
   int ld = img->ld();
   if (ld == 0) ld = img->data_w() * img->d();
   uchar *srcptr = (uchar*)img->array + cy * ld + cx * img->d();
+
+  uchar *dst = fl_read_image(NULL, X, Y, W, H, 0);
+  if (!dst) {
+    fl_draw_image(srcptr, X, Y, W, H, img->d(), ld);
+    return;
+  }
   int srcskip = ld - img->d() * W;
-
-  uchar *dst = new uchar[W * H * 3];
   uchar *dstptr = dst;
 
-  fl_read_image(dst, X, Y, W, H, 0);
-
   uchar srcr, srcg, srcb, srca;
   uchar dstr, dstg, dstb, dsta;
 

src/fl_read_image.cxx

@@ -50,7 +50,7 @@ uchar *fl_read_image(uchar *p, int X, int Y, int w, int h, int alpha) {
     img = Fl_Screen_Driver::traverse_to_gl_subwindows(Fl_Window::current(), X, Y, w, h, NULL);
   }
   int depth = alpha ? 4 : 3;
-  if (img->d() != depth) {
+  if (img && img->d() != depth) {
     uchar *data = new uchar[img->w() * img->h() * depth];
     if (depth == 4) memset(data, alpha, img->w() * img->h() * depth);
     uchar *d = data;