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

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();
   }

web/content.html

@@ -40,6 +40,13 @@ <h3>How to use the generated fonts in LVGL?</h3>
 				</div>
 			</div>
 
+			<div class="form-group row">
+				<label for="name" class="form-label col-md-2">Fallback</label>
+				<div class="col-md-5">
+					<input type="text" name="fallback" id="fallback" class="form-control" placeholder="E.g. lv_font_montserrat_24">
+				</div>
+			</div>
+
 			<div class="form-group row">
 				<label for="height" class="col-md-2 col-form-label">Size</label>
 				<div class="col-md-5">

web/index.js

@@ -60,6 +60,7 @@ document.querySelector('#converterForm').addEventListener('submit', function han
   e.preventDefault();
 
   var _name = document.getElementById('name').value;
+  var _fallback = document.getElementById('fallback').value;
   var _size = document.getElementById('height').value;
   var _bpp = document.getElementById('bpp').value;
   /* eslint-disable max-depth, radix */
@@ -110,7 +111,8 @@ document.querySelector('#converterForm').addEventListener('submit', function han
     lcd_v: document.getElementById('subpixel3').checked,
     use_color_info: document.getElementById('use_color_info').checked,
     format: 'lvgl',
-    output: _name
+    output: _name,
+    lv_fallback: _fallback
   };
 
   convert(args).then(result => {