Add --lv-fallback option, to specify fallback font

Note that the fallback font is declared extern in the generated file.
This is done so that it's possible to use fallback fonts that are not
declared in LVGL's headers.

Fixes: #87

lib/cli.js

@@ -279,6 +279,10 @@ List of characters to copy, belongs to previously declared "--font". Examples:
     help: 'Don\'t shorten "font_info.json" (include pixels data).'
   });
 
+  parser.add_argument('--lv-fallback', {
+    help: 'Variable name of the lvgl font structure to use as fallback for this font. Defaults to NULL.'
+  });
+
   //
   // Process CLI options
   //

lib/writers/lvgl/lv_font.js

@@ -21,6 +21,14 @@ class LvFont extends Font {
       this.font_name = path.basename(options.output, ext);
     }
 
+    if (options.lv_fallback) {
+      this.fallback = '&' + options.lv_fallback;
+      this.fallback_declaration = 'extern const lv_font_t ' + options.lv_fallback + ';\n';
+    } else {
+      this.fallback = 'NULL';
+      this.fallback_declaration = '';
+    }
+
     if (options.bpp === 3 & options.no_compress) {
       throw new AppError('LittlevGL supports "--bpp 3" with compression only');
     }

lib/writers/lvgl/lv_table_head.js

@@ -70,6 +70,7 @@ static lv_font_fmt_txt_dsc_t font_dsc = {
 #endif
 };
 
+${f.fallback_declaration}
 
 /*-----------------
  *  PUBLIC FONT
@@ -93,8 +94,8 @@ lv_font_t ${f.font_name} = {
     .underline_thickness = ${f.src.underlineThickness},
 #endif
     .dsc = &font_dsc,          /*The custom font data. Will be accessed by \`get_glyph_bitmap/dsc\` */
-    .fallback = NULL,
-    .user_data = NULL
+    .fallback = ${f.fallback},
+    .user_data = NULL,
 };
 `.trim();
   }