decompose() should return the original rotation angle and scales.

decompose() would return incorrect rotation angle and scales when the
angle was exactly 180 degrees, due to FPU rounding error.  This commit
fixes it.

This problem would manifest itself when inserting an image into
Calc/Draw, cropping it, and flipping it vertically or rotating it
at exactly 180 degrees.  Before the fix the image would simply disappear.

basegfx/source/matrix/b2dhommatrix.cxx

@@ -320,6 +320,14 @@ namespace basegfx
             // no rotation and shear, copy scale values
             rScale.setX(get(0, 0));
             rScale.setY(get(1, 1));
+
+            // or is there?
+            if( rScale.getX() < 0 && rScale.getY() < 0 )
+            {
+                // there is - 180 degree rotated
+                rScale *= -1;
+                rRotate = 180*F_PI180;
+            }
         }
         else
         {

basegfx/test/basegfx2d.cxx

@@ -36,6 +36,7 @@
 #include "cppunit/extensions/HelperMacros.h"
 
 #include <basegfx/matrix/b2dhommatrix.hxx>
+#include <basegfx/matrix/b2dhommatrixtools.hxx>
 #include <basegfx/polygon/b2dpolygon.hxx>
 #include <basegfx/polygon/b2dpolygontools.hxx>
 #include <basegfx/curve/b2dcubicbezier.hxx>
@@ -927,6 +928,22 @@ public:
         CPPUNIT_ASSERT_MESSAGE("decompose: error test I2", impDecomposeComposeTest(-fSX, fSY, tan(-fS), -fR));
         CPPUNIT_ASSERT_MESSAGE("decompose: error test I3", impDecomposeComposeTest(fSX, -fSY, tan(-fS), -fR));
         CPPUNIT_ASSERT_MESSAGE("decompose: error test I4", impDecomposeComposeTest(-fSX, -fSY, tan(-fS), -fR));
+
+        // cover special case of 180 degree rotation
+        B2DHomMatrix aTest=tools::createScaleShearXRotateTranslateB2DHomMatrix(
+            6425,3938,
+            0,
+            180*F_PI180,
+            10482,4921);
+        // decompose that matrix
+        B2DTuple aDScale;
+        B2DTuple aDTrans;
+        double fDRot;
+        double fDShX;
+        aTest.decompose(aDScale, aDTrans, fDRot, fDShX);
+        CPPUNIT_ASSERT_MESSAGE("decompose: error test J1", aDScale.getX() == 6425 && aDScale.getY() == 3938);
+        CPPUNIT_ASSERT_MESSAGE("decompose: error test J1", aDTrans.getX() == 10482 && aDTrans.getY() == 4921);
+        CPPUNIT_ASSERT_MESSAGE("decompose: error test J1", fDRot == 180*F_PI180);
     }
 
     // Change the following lines only, if you add, remove or rename