Only compile png files, also add bgb testing

scripts/build.js

@@ -90,6 +90,7 @@ Object.entries(GAME_STRINGS).forEach(entry => {
 
 // Gen imagery
 fs.readdirSync(DIR_IMAGES).forEach(img => {
+  if(!img.endsWith(".png")) return;
   const { fileC, fileH } = png2gb(
     path.join(DIR_IMAGES, img), DIR_GENERATED,
     path.basename(img, '.png').toUpperCase()