Fix UnrollPlanarWordsPremul() skipping only 1 plane instead of Extra planes

UnrollPlanarWordsPremul() in src/cmspack.c advances accum by a single
Stride when ExtraFirst is set, regardless of how many extra (non-color)
planes are declared via T_EXTRA(). For formats with Extra > 1 (e.g. two
extra planes ahead of the color planes), this shifts every subsequent
color-channel read one plane early and leaves the last color channel
unread, corrupting the unrolled pixel.

The chunky sibling UnrollAnyWordsPremul() had the identical bug, fixed
in a prior merge by adding an Extra local (T_EXTRA(info->InputFormat))
and multiplying it into the pre-skip offset. This applies the same fix
to the planar function: add the missing Extra local and change the
ExtraFirst pre-skip from `accum += Stride` to `accum += Extra * Stride`.

Verified with a formatter-dispatch harness exercising
COLORSPACE_SH(PT_CMYK)|CHANNELS_SH(4)|BYTES_SH(2)|PLANAR_SH(1)|
EXTRA_SH(2)|PREMUL_SH(1)|DOSWAP_SH(1): before the fix, wIn came out as
{0x5555,0x4444,0x3333,0x2222} instead of the correct
{0x6666,0x5555,0x4444,0x3333}, with the K channel never read. After
the fix all 5 regression cases (Extra=1/2/3, both ExtraFirst settings)
pass with no change to previously-correct behavior.

Author: 94xhn

src/cmspack.c

@@ -659,15 +659,16 @@ cmsUInt8Number* UnrollPlanarWordsPremul(CMSREGISTER _cmsTRANSFORM* info,
     cmsUInt32Number SwapFirst = T_SWAPFIRST(info->InputFormat);
     cmsUInt32Number Reverse= T_FLAVOR(info ->InputFormat);
     cmsUInt32Number SwapEndian = T_ENDIAN16(info -> InputFormat);
+    cmsUInt32Number Extra = T_EXTRA(info -> InputFormat);
     cmsUInt32Number i;
     cmsUInt32Number ExtraFirst = DoSwap ^ SwapFirst;
     cmsUInt8Number* Init = accum;
-    
+
     cmsUInt16Number alpha = (ExtraFirst ? ((cmsUInt16Number*)accum)[0] : ((cmsUInt16Number*)accum)[nChan * Stride / 2]);
     cmsUInt32Number alpha_factor = _cmsToFixedDomain(alpha);
 
     if (ExtraFirst) {
-        accum += Stride;
+        accum += Extra * Stride;
     }
 
     for (i=0; i < nChan; i++) {