Add a C API copy method for font_desc objects

Add a renderer.font's method "copy" to clone the font object
by specifying an optional new size. In the size is not given
the size of the original object is used.

Should fulfill the request from issue #288.

src/api/renderer_font.c

@@ -55,6 +55,21 @@ static int f_load(lua_State *L) {
 }
 
 
+static int f_copy(lua_State *L) {
+  FontDesc *self = luaL_checkudata(L, 1, API_TYPE_FONT);
+  float size;
+  if (lua_gettop(L) >= 2) {
+    size = luaL_checknumber(L, 2);
+  } else {
+    size = self->size;
+  }
+  FontDesc *new_font_desc = lua_newuserdata(L, font_desc_alloc_size(self->filename));
+  font_desc_init(new_font_desc, self->filename, size, self->options);
+  luaL_setmetatable(L, API_TYPE_FONT);
+  return 1;
+}
+
+
 static int f_set_tab_size(lua_State *L) {
   FontDesc *self = luaL_checkudata(L, 1, API_TYPE_FONT);
   int n = luaL_checknumber(L, 2);
@@ -123,6 +138,7 @@ static int f_set_size(lua_State *L) {
 static const luaL_Reg lib[] = {
   { "__gc",               f_gc                 },
   { "load",               f_load               },
+  { "copy",               f_copy               },
   { "set_tab_size",       f_set_tab_size      },
   { "get_width",          f_get_width          },
   { "get_width_subpixel", f_get_width_subpixel },