Improve the CLI interface of decode.py

Check argc properly + optionally allow specifying of an output path
instead of always deriving it from input image name.

decode.py

@@ -26,6 +26,9 @@ from PIL import Image
 
 import sys
 
+if not len(sys.argv) in [2, 3]:
+    print(f"Usage: {sys.argv[0]} <input.rif> [output.png]")
+
 # RGB, RGBA
 frm_sizes = [3, 4]
 frm_names = ["RGB", "RGBA"]
@@ -41,4 +44,7 @@ height = (len(data)//frm_size)//width
 
 print(width, height)
 img = Image.frombuffer(frm_names[formt], (width, height), data, "raw", frm_names[formt], 0, 1)
-img.save('.'.join(sys.argv[1].split(".")[:1])+".png")
+if len(sys.argv) == 2:
+    img.save('.'.join(sys.argv[1].split(".")[:1])+".png")
+elif len(sys.argv) == 3:
+    img.save(sys.argv[2])