Implement optional font rendering options

The user can now choose antialiasing grayscale and subpixel and hinting
non, slight or full.

src/api/renderer_font.c

@@ -6,9 +6,43 @@
 static int f_load(lua_State *L) {
   const char *filename  = luaL_checkstring(L, 1);
   float size = luaL_checknumber(L, 2);
+  unsigned int font_options = 0;
+  if (lua_gettop(L) > 2 && lua_istable(L, 3)) {
+    lua_getfield(L, 3, "antialiasing");
+    if (lua_isstring(L, -1)) {
+      const char *antialiasing = lua_tostring(L, -1);
+      if (antialiasing) {
+        if (strcmp(antialiasing, "grayscale") == 0) {
+          font_options |= RenFontGrayscale;
+        } else if (strcmp(antialiasing, "subpixel") == 0) {
+          font_options |= RenFontSubpixel;
+        } else {
+          return luaL_error(L, "error in renderer.font.load, unknown antialiasing option: \"%s\"", antialiasing);
+        }
+      }
+    }
+    lua_pop(L, 1);
+
+    lua_getfield(L, 3, "hinting");
+    if (lua_isstring(L, -1)) {
+      const char *hinting = lua_tostring(L, -1);
+      if (hinting) {
+        if (strcmp(hinting, "slight") == 0) {
+          font_options |= RenFontHintingSlight;
+        } else if (strcmp(hinting, "none") == 0) {
+          font_options |= RenFontHintingNone;
+        } else if (strcmp(hinting, "full") == 0) {
+          font_options |= RenFontHintingFull;
+        } else {
+          return luaL_error(L, "error in renderer.font.load, unknown hinting option: \"%s\"", hinting);
+        }
+      }
+    }
+    lua_pop(L, 1);
+  }
   RenFont **self = lua_newuserdata(L, sizeof(*self));
   luaL_setmetatable(L, API_TYPE_FONT);
-  *self = ren_load_font(filename, size);
+  *self = ren_load_font(filename, size, font_options);
   if (!*self) { luaL_error(L, "failed to load font"); }
   return 1;
 }