FBT: Only format images in firmware, not ext apps

It was formatting demo screenshots too
There might a better way to detect only icons, but not formatting also means less merge conflicts

CHANGELOG.md

@@ -14,6 +14,7 @@
 - FBT:
   - OFW: Add `-Wundef` to compiler options (by @hedger)
   - OFW: Ensure that all images conform specification (by @skyhawkillusions & @hedger)
+  - Don't format images in external apps folder, only firmware (by @Willy-JL)
 
 ### Updated:
 - Apps:

SConstruct

@@ -326,6 +326,7 @@ firmware_env.Append(
     IMG_LINT_SOURCES=[
         # Image assets
         "applications",
+        "!applications/external",
         "assets",
     ],
 )

scripts/imglint.py

@@ -57,10 +57,15 @@ class ImageLint(App):
     def _gather_images(self, folders):
         images = []
         for folder in folders:
-            for dirpath, _, filenames in os.walk(folder):
+            exclude = folder.startswith("!")
+            for dirpath, _, filenames in os.walk(folder.removeprefix("!")):
                 for filename in filenames:
                     if self.is_file_an_icon(filename):
-                        images.append(os.path.join(dirpath, filename))
+                        filepath = os.path.join(dirpath, filename)
+                        if exclude:
+                            images.remove(filepath)
+                        else:
+                            images.append(filepath)
         return images
 
     def is_file_an_icon(self, filename):