Clear old CameraParameters in AffineBasedEstimator

AffineBasedEstimator crashed when called with an existing CameraParameters.
This happens e.g. when using Stitcher in SCANS mode.
CameraraParameters is now cleared before any calculation is executed.

modules/stitching/src/motion_estimators.cpp

@@ -199,7 +199,7 @@ bool AffineBasedEstimator::estimate(const std::vector<ImageFeatures> &features,
                                     const std::vector<MatchesInfo> &pairwise_matches,
                                     std::vector<CameraParams> &cameras)
 {
-    cameras.resize(features.size());
+    cameras.assign(features.size(), CameraParams());
     const int num_images = static_cast<int>(features.size());
 
     // find maximum spaning tree on pairwise matches