get vertices count without jumping through hoops

Change-Id: I0cab6b2f9fffc8cf9fc5059b8dc48fa9407e42b5

slideshow/source/engine/opengl/TransitionImpl.cxx

@@ -1998,9 +1998,8 @@ void GlitterTransition::prepareTransition( sal_Int32 glLeavingSlideTex, sal_Int3
 
     // Upload the center of each hexagon.
     const Primitive& primitive = getScene().getLeavingSlide()[0];
-    int nbVertices = primitive.getVerticesByteSize() / sizeof(Vertex);
     std::vector<ThreeFloats> vertices;
-    for (int i = 2; i < nbVertices; i += 18) {
+    for (int i = 2; i < primitive.getVerticesCount(); i += 18) {
         const glm::vec3& center = primitive.getVertex(i);
         for (int j = 0; j < 18; ++j)
             vertices.push_back({center.x, center.y, center.z});

slideshow/source/engine/opengl/TransitionImpl.hxx

@@ -33,6 +33,7 @@
 
 #include <GL/glew.h>
 
+#include <limits>
 #include <memory>
 #include <vector>
 
@@ -335,6 +336,12 @@ public:
     */
     const glm::vec3& getVertex(int n) const {return Vertices[n].position;}
 
+    int getVerticesCount() const
+    {
+        assert(Vertices.size() < std::numeric_limits<int>::max());
+        return int(unsigned(Vertices.size()));
+    }
+
     /** accessor for the size of the vertices data
 
         @return