decode threads is a module constant (#76)

Also adds num_threads to the signatures in pyi

pillow_jxl/JpegXLImagePlugin.py

@@ -8,6 +8,7 @@
 from pillow_jxl import Decoder, Encoder
 
 _VALID_JXL_MODES = {"RGB", "RGBA", "L", "LA"}
+DECODE_THREADS = -1 # -1 detect available cpu cores, 0 disables parallelism
 
 
 def _accept(data):
@@ -26,7 +27,7 @@ class JXLImageFile(ImageFile.ImageFile):
 
     def _open(self):
         self.fc = self.fp.read()
-        self._decoder = Decoder()
+        self._decoder = Decoder(num_threads=DECODE_THREADS)
 
         self.jpeg, self._jxlinfo, self._data, icc_profile = self._decoder(self.fc)
         # FIXME (Isotr0py): Maybe slow down jpeg reconstruction

pillow_jxl/__init__.pyi

@@ -15,7 +15,8 @@ class Encoder:
                  parallel: bool = True,
                  has_alpha: bool = False,
                  lossless: bool = True,
-                 quality: float = 0.0): ...
+                 quality: float = 0.0,
+                 num_threads: int = -1): ...
 
     def __call__(self, data: bytes, width: int, height: int, jpeg_encode: bool) -> bytes: ...
     '''
@@ -37,7 +38,7 @@ class Decoder:
             parallel(`bool`): enable parallel decoding
     '''
 
-    def __init__(self, parallel: bool = True): ...
+    def __init__(self, num_threads: int = -1): ...
 
     def __call__(self, data: bytes) -> (bool, ImageInfo, bytes): ...
     '''