Doc: "MCU block" = "iMCU" or "MCU"

The JPEG-1 spec never uses the term "MCU block".  That term is rarely
used in other literature to describe the equivalent of an MCU in an
interleaved JPEG image, but the libjpeg documentation uses "iMCU" to
describe the same thing.  "iMCU" is a better term, since the equivalent
of an interleaved MCU can contain multiple DCT blocks (or samples in
lossless mode) that are only grouped together if the image is
interleaved.

In the case of restart markers, "MCU block" was used in the libjpeg
documentation instead of "MCU", but "MCU" is more accurate and less
confusing.  (The restart interval is literally in MCUs, where one MCU
is one data unit in a non-interleaved JPEG image and multiple data units
in a multi-component interleaved JPEG image.)

In the case of 9b704f96b2, the issue was
actually with progressive JPEG images exactly two DCT blocks wide, not
two MCU blocks wide.

This commit also defines "MCU" and "MCU row" in the description of the
various restart marker options/parameters.  Although an MCU row is
technically always a row of samples in lossless mode, "sample row" was
confusing, since it is used in other places to describe a row of samples
for a single component (whereas an MCU row in a typical lossless JPEG
image consists of a row of interleaved samples for all components.)

turbojpeg.c

@@ -1940,7 +1940,7 @@ DLLEXPORT int tj3SetCroppingRegion(tjhandle handle, tjregion croppingRegion)
   if (croppingRegion.x %
       TJSCALED(tjMCUWidth[this->subsamp], this->scalingFactor) != 0)
     THROWI("The left boundary of the cropping region (%d) is not\n"
-           "divisible by the scaled MCU width (%d)",
+           "divisible by the scaled iMCU width (%d)",
            croppingRegion.x,
            TJSCALED(tjMCUWidth[this->subsamp], this->scalingFactor));
   if (croppingRegion.w == 0)