Fix regression calling fl_font() w/o draw context

According to the documentation fl_font(face, size) may be called
"outside a draw context if necessary to call fl_width()".

This worked in 1.3.5 but did not in 1.4 (so far). I reworded the
docs to make clear that other measurement functions can be called
as well and refactored the code to make sure that fl_font(face, size)
will open the display if necessary.

FL/fl_draw.H

@@ -574,16 +574,11 @@ FL_EXPORT void fl_rescale_offscreen(Fl_Offscreen &ctx);
 /* NOTE: doxygen comments here to avoid triplication in os-specific sources */
 
 // Fonts:
-/**
+/*
   Sets the current font, which is then used in various drawing routines.
-  You may call this outside a draw context if necessary to call fl_width(),
+  Implemented and documented in src/Fl_Graphics_Driver.cxx
-  but on X this will open the display.
-
-  The font is identified by a \p face and a \p size.
-  The size of the font is measured in pixels and not "points".
-  Lines should be spaced \p size pixels apart or more.
 */
-inline void fl_font(Fl_Font face, Fl_Fontsize fsize) { fl_graphics_driver->font(face,fsize); }
+void fl_font(Fl_Font face, Fl_Fontsize fsize);
 
 /**
   Returns the \p face set by the most recent call to fl_font().

src/Fl_Graphics_Driver.cxx

@@ -557,6 +557,22 @@ void Fl_Graphics_Driver::rtl_draw(const char *str, int nChars, int x, int y) {
 /** Returns non-zero if the graphics driver possesses the \p feature */
 int Fl_Graphics_Driver::has_feature(driver_feature feature) { return 0; }
 
+/**
+  Sets the current font, which is then used in various drawing routines.
+  You may call this outside a draw context if necessary to measure text,
+  for instance by calling fl_width(), fl_measure(), or fl_text_extents(),
+  but on X this will open the display.
+
+  The font is identified by a \p face and a \p size.
+  The size of the font is measured in pixels and not "points".
+  Lines should be spaced \p size pixels apart or more.
+*/
+void fl_font(Fl_Font face, Fl_Fontsize fsize) {
+  if (!fl_graphics_driver)
+    fl_open_display();
+  fl_graphics_driver->font(face, fsize);
+}
+
 /** see fl_font(Fl_Font, Fl_Fontsize) */
 void Fl_Graphics_Driver::font(Fl_Font face, Fl_Fontsize fsize) {font_ = face; size_ = fsize;}