Mostly fix problem of offset with cursor positioning when using mouse

In order to get right the cursor position on text on mouse clicks it is
needed to take into account text's subpixel positioning.

This fix mostly corrects the problem but cursor positioning is still
somewhat inaccurate for long lines due to repeated commands to draw a text
along a line. Repeated draw text calls make the subpixel information
lost and small errors will add-up.

src/api/renderer_font.c

@@ -66,7 +66,24 @@ static int f_gc(lua_State *L) {
 static int f_get_width(lua_State *L) {
   RenFont **self = luaL_checkudata(L, 1, API_TYPE_FONT);
   const char *text = luaL_checkstring(L, 2);
-  lua_pushnumber(L, ren_get_font_width(*self, text) );
+  int subpixel_scale;
+  int w = ren_get_font_width(*self, text, &subpixel_scale);
+  lua_pushnumber(L, ren_font_subpixel_round(w, subpixel_scale, 0));
+  return 1;
+}
+
+
+static int f_subpixel_scale(lua_State *L) {
+  RenFont **self = luaL_checkudata(L, 1, API_TYPE_FONT);
+  lua_pushnumber(L, ren_get_font_subpixel_scale(*self));
+  return 1;
+}
+
+
+static int f_get_width_subpixel(lua_State *L) {
+  RenFont **self = luaL_checkudata(L, 1, API_TYPE_FONT);
+  const char *text = luaL_checkstring(L, 2);
+  lua_pushnumber(L, ren_get_font_width(*self, text, NULL));
   return 1;
 }
 
@@ -79,11 +96,13 @@ static int f_get_height(lua_State *L) {
 
 
 static const luaL_Reg lib[] = {
-  { "__gc",          f_gc            },
-  { "load",          f_load          },
-  { "set_tab_width", f_set_tab_width },
-  { "get_width",     f_get_width     },
-  { "get_height",    f_get_height    },
+  { "__gc",               f_gc                 },
+  { "load",               f_load               },
+  { "set_tab_width",      f_set_tab_width      },
+  { "get_width",          f_get_width          },
+  { "get_width_subpixel", f_get_width_subpixel },
+  { "get_height",         f_get_height         },
+  { "subpixel_scale",     f_subpixel_scale     },
   { NULL, NULL }
 };