Fix GetSubBitmap() returned value size when rounding is required

When using non-integer scale factor, we need to round the result of the
multiplication by it instead of truncating it for consistency with the
behaviour of wxBitmap::CreateWithDIPSize() and the other places where we
do this, including wxOSX version of this function.

Add a simple unit test checking that this works correctly (it would fail
with wxGTK before).

src/gtk/bitmap.cpp

@@ -1062,7 +1062,7 @@ wxBitmap wxBitmap::GetSubBitmap(const wxRect& r) const
     const wxBitmapRefData* bmpData = M_BMPDATA;
 #ifdef __WXGTK3__
     const double s = bmpData->m_scaleFactor;
-    const wxRect rect(int(r.x * s), int(r.y * s), int(r.width * s), int(r.height * s));
+    const wxRect rect(wxRound(r.x * s), wxRound(r.y * s), wxRound(r.width * s), wxRound(r.height * s));
 #else
     const wxRect& rect = r;
 #endif

tests/graphics/bitmap.cpp

@@ -1826,4 +1826,32 @@ TEST_CASE("Bitmap::ScaleFactor", "[bitmap][dc][scale]")
     CHECK( bmp3.GetSize() == wxSize(16, 16) );
 }
 
+TEST_CASE("wxBitmap::GetSubBitmap", "[bitmap]")
+{
+    // Make the logical size odd to test correct rounding.
+    const double scale = 1.5;
+    const wxSize sizeLog(15, 15);
+    const wxSize sizePhy(23, 23);
+
+    // Prepare the main bitmap.
+    wxBitmap bmp;
+    bmp.CreateWithDIPSize(sizeLog, scale);
+    CHECK( bmp.GetDIPSize() == sizeLog );
+    CHECK( bmp.GetSize() == sizePhy );
+    CHECK( bmp.GetScaleFactor() == scale );
+
+    // Extracting sub-bitmap of the entire bitmap size should return the bitmap
+    // of the same size.
+#if wxHAS_DPI_INDEPENDENT_PIXELS
+    const wxRect rectAll(wxPoint(0, 0), sizeLog);
+#else
+    const wxRect rectAll(wxPoint(0, 0), sizePhy);
+#endif
+
+    const wxBitmap sub = bmp.GetSubBitmap(rectAll);
+    CHECK( sub.GetDIPSize() == sizeLog );
+    CHECK( sub.GetSize() == sizePhy );
+    CHECK( sub.GetScaleFactor() == scale );
+}
+
 #endif // ports with scaled bitmaps support