added cross dissolve transitions (but they're uneditable)

effects/crossdissolvetransition.cpp

@@ -0,0 +1,9 @@
+#include "transition.h"
+
+#include <QOpenGLFunctions>
+
+void CrossDissolveTransition::process_transition(float progress) {
+    float color[4];
+    glGetFloatv(GL_CURRENT_COLOR, color);
+    glColor4f(1.0, 1.0, 1.0, color[3]*progress);
+}