Load fonts asynchronously on a bg task

This saves a second or two from bootup; AFAICT this *mostly* reclaims
the dynamic fonts boot time regression.

lib/luavgl/src/font.c

@@ -1,49 +1,23 @@
 #include "luavgl.h"
 #include "private.h"
 
-static char *to_lower(char *str)
-{
-  for (char *s = str; *s; ++s)
-    *s = *s >= 'A' && *s <= 'Z' ? *s | 0x60 : *s;
-  return str;
-}
-
-static char *luavgl_strchr(const char *s, char c)
-{
-  while (*s) {
-    if (c == *s) {
-      return (char *)s;
-    }
-    s++;
-  }
-  return NULL;
-}
-
-/**
- * Dynamic font family fallback is not supported.
- * The fallback only happen when font creation fails and continue to try next
- * one. Fallback logic in lvgl is supposed to be system wide.
- *
- * lvgl.Font("MiSansW medium, montserrat", 24, "normal")
- */
 static int luavgl_font_create(lua_State *L)
 {
-
   if (!lua_isstring(L, 1)) {
     return luaL_argerror(L, 1, "expect string");
   }
-  const char *name = lua_tostring(L, 1);
-  const lv_font_t *font = NULL;
+  if (!lua_isfunction(L, 2)) {
+    return luaL_argerror(L, 1, "expect function");
+  }
 
   luavgl_ctx_t *ctx = luavgl_context(L);
-  if (ctx->make_font) {
-    font = ctx->make_font(name);
+  if (!ctx->make_font) {
+    return luaL_error(L, "cannot create font");
   }
 
-  if (font) {
-    lua_pushlightuserdata(L, (void *)font);
-    return 1;
-  }
+  const char *name = lua_tostring(L, 1);
+  int cb_ref = luaL_ref(L, LUA_REGISTRYINDEX);
+  ctx->make_font(L, name, cb_ref);
 
-  return luaL_error(L, "cannot create font");
+  return 0;
 }

lib/luavgl/src/fs.c

@@ -1,6 +1,17 @@
 #include "luavgl.h"
 #include "private.h"
 
+static char *luavgl_strchr(const char *s, char c)
+{
+  while (*s) {
+    if (c == *s) {
+      return (char *)s;
+    }
+    s++;
+  }
+  return NULL;
+}
+
 typedef struct luavgl_fs_s {
   lv_fs_file_t file;
   bool closed; /* userdata exists but lv_fs has been closed */

lib/luavgl/src/luavgl.h

@@ -12,7 +12,7 @@
 extern "C" {
 #endif
 
-typedef const lv_font_t *(*make_font_cb)(const char *);
+typedef void (*make_font_cb)(lua_State *L, const char *, int cb);
 typedef void (*delete_font_cb)(const lv_font_t *);
 typedef int (*luavgl_pcall_t)(lua_State *L, int nargs, int nresults);