Derive PCX plane count from the image, not the line width

Tests/test_file_pcx.py

@@ -118,6 +118,14 @@ def test_1px_width(tmp_path: Path) -> None:
     _roundtrip(tmp_path, im)
 
 
+def test_rgb_odd_width(tmp_path: Path) -> None:
+    # An RGB (multi-plane) width whose even-padded stride differs from the
+    # width must still separate the colour planes correctly. Distinct channel
+    # values expose a misaligned split.
+    im = Image.new("RGB", (3, 3), (0x11, 0x22, 0x33))
+    _roundtrip(tmp_path, im)
+
+
 def test_large_count(tmp_path: Path) -> None:
     im = Image.new("L", (256, 1))
     px = im.load()

src/libImaging/PcxDecode.c

@@ -69,10 +69,12 @@ ImagingPcxDecode(Imaging im, ImagingCodecState state, UINT8 *buf, Py_ssize_t byt
                 stride = state->bytes / state->bits;
             } else {
                 xsize = state->xsize;
-                bands = state->bytes / state->xsize;
-                if (bands != 0) {
-                    stride = state->bytes / bands;
-                }
+                // state->bytes is planes * stride; derive the stride from the
+                // known band count rather than guessing it from state->xsize,
+                // which picks the wrong split when xsize != stride (e.g. an
+                // odd-width RGB line padded to an even stride).
+                bands = im->bands;
+                stride = state->bytes / bands;
             }
             if (stride > xsize) {
                 int i;