Remove padding between interleaved PCX palette data (#9005)

Author: radarhere

Tests/images/p_4_planes.pcx

@@ -37,6 +37,11 @@ def test_sanity(tmp_path: Path) -> None:
         im.save(f)
 
 
+def test_p_4_planes() -> None:
+    with Image.open("Tests/images/p_4_planes.pcx") as im:
+        assert im.getpixel((0, 0)) == 3
+
+
 def test_bad_image_size() -> None:
     with open("Tests/images/pil184.pcx", "rb") as fp:
         data = fp.read()

Tests/test_file_pcx.py

@@ -60,15 +60,25 @@ ImagingPcxDecode(Imaging im, ImagingCodecState state, UINT8 *buf, Py_ssize_t byt
         }
 
         if (state->x >= state->bytes) {
-            if (state->bytes % state->xsize && state->bytes > state->xsize) {
-                int bands = state->bytes / state->xsize;
-                int stride = state->bytes / bands;
+            int bands;
+            int xsize = 0;
+            int stride = 0;
+            if (state->bits == 2 || state->bits == 4) {
+                xsize = (state->xsize + 7) / 8;
+                bands = state->bits;
+                stride = state->bytes / state->bits;
+            } else {
+                xsize = state->xsize;
+                bands = state->bytes / state->xsize;
+                if (bands != 0) {
+                    stride = state->bytes / bands;
+                }
+            }
+            if (stride > xsize) {
                 int i;
                 for (i = 1; i < bands; i++) {  // note -- skipping first band
                     memmove(
-                        &state->buffer[i * state->xsize],
-                        &state->buffer[i * stride],
-                        state->xsize
+                        &state->buffer[i * xsize], &state->buffer[i * stride], xsize
                     );
                 }
             }