fix bug where zooming out to 0% and trying to enlarge it gets stuck a…

…t 0%

This was due to the possibility that the new X and Y dimensions are still the same (due to rounding) but the zoom is increasing.
The old code would restore the zoom value, which would cause a state that you can't zoom in.
The new code takes into account that, if you're zooming out, it'll restore the old zoom value, but if zooming in, allow the factor to continue to increase, even if the newX and newY don't change.

Closes #159

src/JPEGView/MainDlg.cpp

@@ -455,7 +455,7 @@ LRESULT CMainDlg::OnPaint(UINT /*uMsg*/, WPARAM /*wParam*/, LPARAM /*lParam*/, B
 
 #ifdef DEBUG
 	CString a; a.Format(_T("client rect w/h pix: %d %d = %d\n"), m_clientRect.Width(), m_clientRect.Height(), m_clientRect.Width() * m_clientRect.Height());
-	::OutputDebugString(a);
+	//::OutputDebugString(a);
 #endif
 
 	std::list<CRect> excludedClippingRects;
@@ -2648,13 +2648,37 @@ void CMainDlg::PerformZoom(double dValue, bool bExponent, bool bZoomToMouse, boo
 		nNewYSize = (int)(m_pCurrentImage->OrigHeight() * m_dZoom + 0.5);
 	}
 
-	// because we've increased the maximum zoom allowed, only actually perform the zoom if the old and new dimensions differ
-	// the float arithmetic doesn't always come up with the same answer
-	if (nOldXSize == nNewXSize && nOldYSize == nNewYSize) {
-		m_dZoom = dOldZoom;  // restore previous zoom value, then do nothing
+#ifdef DEBUG
+	CString a; a.Format(_T("dZoom dOldZoom: %f %f\n"), m_dZoom, dOldZoom);
+	::OutputDebugString(a);
+#endif
+
+	// because we've increased/decreased to the maximum zoom allowed,
+	// only actually perform the zoom if the old and new dimensions differ,
+	// or at the edge of allowable max dimension (maximum 65535 pixels)
+	if (nNewXSize == 65535 || nNewYSize == 65535) {
+		// zooming in does not require restoring the previous zoom value
+		// because the code which checks the 65535 will re-scale the zoom factor to ensure it never exceeds 65535
 		return;
 	}
 
+	// the float arithmetic doesn't always come up with the same answer, but the new sizes can be directly compared
+	if (nNewXSize == nOldXSize && nNewYSize == nOldYSize) {
+		// because there's rounding errors, it is possible to get stuck zooming out from fractional zoom,
+		// due to the new/old size being the same, but the zoom factor still changing
+		// hence, only reset the zoom value IF when zooming out (aka, it is getting smaller)
+		//
+		// when zooming in, do not set the old zoom value so that it can "escape" the initial few steps where
+		// the zoom ratio has changed, but the size is still the same, due to rounding errors
+		if (bExponent && dValue < 0) {
+			// zooming out
+			// NOTE: this gets triggered on pauseAtZoom
+			m_dZoom = dOldZoom;  // restore previous zoom value
+		}
+
+		return; // then do nothing
+	}
+
 
 	if (bZoomToMouse) {
 		// zoom to mouse