Simplify code in FpxImagePlugin.py (#9423)

Author: hugovk

src/PIL/FpxImagePlugin.py

@@ -141,7 +141,7 @@ def _open_subimage(self, index: int = 1, subimage: int = 0) -> None:
 
         size = i32(s, 4), i32(s, 8)
         # tilecount = i32(s, 12)
-        tilesize = i32(s, 16), i32(s, 20)
+        xtile, ytile = i32(s, 16), i32(s, 20)
         # channels = i32(s, 24)
         offset = i32(s, 28)
         length = i32(s, 32)
@@ -156,7 +156,6 @@ def _open_subimage(self, index: int = 1, subimage: int = 0) -> None:
 
         x = y = 0
         xsize, ysize = size
-        xtile, ytile = tilesize
         self.tile = []
 
         for i in range(0, len(s), length):
@@ -224,7 +223,7 @@ def _open_subimage(self, index: int = 1, subimage: int = 0) -> None:
                 msg = "unknown/invalid compression"
                 raise OSError(msg)
 
-            x = x + xtile
+            x += xtile
             if x >= xsize:
                 x, y = 0, y + ytile
                 if y >= ysize: