DebugTools: improve CompareImage failure message.

The comparison is <=, so if it passes, it's not "below".

doc/snippets/debugtools-compareimage.ansi

@@ -1,7 +1,7 @@
 Starting ProcessingTest with 1 test cases...
   FAIL [1] process() at …/debugtools-compareimage.cpp on line 77
         Images actual and expected have max delta above threshold, actual 189
-        but at most 170 expected. Mean delta 13.5776 is below threshold 96.
+        but at most 170 expected. Mean delta 13.5776 is within threshold 96.
         Delta image:
           |                                |
           |                                |

src/Magnum/DebugTools/CompareImage.cpp

@@ -611,12 +611,12 @@ void ImageComparatorBase::printMessage(TestSuite::ComparisonStatusFlags, Debug&
         else if(_state->result == Result::AboveMaxThreshold)
             out << "max delta above threshold, actual" << _state->max
                 << "but at most" << _state->maxThreshold
-                << "expected. Mean delta" << _state->mean << "is below threshold"
+                << "expected. Mean delta" << _state->mean << "is within threshold"
                 << _state->meanThreshold << Debug::nospace << ".";
         else if(_state->result == Result::AboveMeanThreshold)
             out << "mean delta above threshold, actual" << _state->mean
                 << "but at most" << _state->meanThreshold
-                << "expected. Max delta" << _state->max << "is below threshold"
+                << "expected. Max delta" << _state->max << "is within threshold"
                 << _state->maxThreshold << Debug::nospace << ".";
         else CORRADE_ASSERT_UNREACHABLE(); /* LCOV_EXCL_LINE */
 

src/Magnum/DebugTools/Test/CompareImageTest.cpp

@@ -588,7 +588,7 @@ void CompareImageTest::compareAboveMaxThreshold() {
     }
 
     CORRADE_COMPARE(out.str(),
-        "Images a and b have max delta above threshold, actual 39 but at most 30 expected. Mean delta 18.5 is below threshold 20. Delta image:\n"
+        "Images a and b have max delta above threshold, actual 39 but at most 30 expected. Mean delta 18.5 is within threshold 20. Delta image:\n"
         "          |?M|\n"
         "        Pixels above max/mean threshold:\n"
         "          [1,1] #abcd85, expected #abcdfa (Δ = 39)\n");
@@ -607,7 +607,7 @@ void CompareImageTest::compareAboveMeanThreshold() {
     }
 
     CORRADE_COMPARE(out.str(),
-        "Images a and b have mean delta above threshold, actual 18.5 but at most 18 expected. Max delta 39 is below threshold 50. Delta image:\n"
+        "Images a and b have mean delta above threshold, actual 18.5 but at most 18 expected. Max delta 39 is within threshold 50. Delta image:\n"
         "          |?M|\n"
         "        Pixels above max/mean threshold:\n"
         "          [1,1] #abcd85, expected #abcdfa (Δ = 39)\n"
@@ -684,7 +684,7 @@ void CompareImageTest::compareSpecialsMeanOnly() {
        NaNs. This is *not* a libc++ thing, tho -- libc++ on Linux prints signed NaNs. */
     #if defined(CORRADE_TARGET_APPLE) || defined(CORRADE_TARGET_ANDROID) || defined(CORRADE_TARGET_EMSCRIPTEN) || defined(__MINGW32__)
     CORRADE_COMPARE(out.str(),
-        "Images a and b have mean delta above threshold, actual nan but at most 0.5 expected. Max delta 3.1 is below threshold 15. Delta image:\n"
+        "Images a and b have mean delta above threshold, actual nan but at most 0.5 expected. Max delta 3.1 is within threshold 15. Delta image:\n"
         "          |MMMM M ,M|\n"
         "        Pixels above max/mean threshold:\n"
         "          [5,0] Vector(-inf), expected Vector(inf) (Δ = inf)\n"
@@ -697,7 +697,7 @@ void CompareImageTest::compareSpecialsMeanOnly() {
     /* MSVC prints -nan(ind) instead of ±nan. But only sometimes. */
     #elif defined(CORRADE_TARGET_WINDOWS) && defined(_MSC_VER)
     CORRADE_COMPARE(out.str(),
-        "Images a and b have mean delta above threshold, actual -nan(ind) but at most 0.5 expected. Max delta 3.1 is below threshold 15. Delta image:\n"
+        "Images a and b have mean delta above threshold, actual -nan(ind) but at most 0.5 expected. Max delta 3.1 is within threshold 15. Delta image:\n"
         "          |MMMM M ,M|\n"
         "        Pixels above max/mean threshold:\n"
         "          [5,0] Vector(-inf), expected Vector(inf) (Δ = inf)\n"
@@ -710,7 +710,7 @@ void CompareImageTest::compareSpecialsMeanOnly() {
     /* Linux */
     #else
     CORRADE_COMPARE(out.str(),
-        "Images a and b have mean delta above threshold, actual -nan but at most 0.5 expected. Max delta 3.1 is below threshold 15. Delta image:\n"
+        "Images a and b have mean delta above threshold, actual -nan but at most 0.5 expected. Max delta 3.1 is within threshold 15. Delta image:\n"
         "          |MMMM M ,M|\n"
         "        Pixels above max/mean threshold:\n"
         "          [5,0] Vector(-inf), expected Vector(inf) (Δ = inf)\n"