Add gradient animation steps

makes linear gradients in both fills/shapes animatable!

(both the steps and the start/end xy coords)

Diffs=
210e0ab14 Add gradient animation steps (#4451)

.rive_head

diff --git a/.rive_head b/.rive_head
index bee22c8..6e18a36 100644
--- a/.rive_head
+++ b/.rive_head
@@ -1 +1 @@
-5d661f39a523edf569d9b8b7deed85780309b096
+210e0ab148b628d05400073230bb70a65e787772

include/rive/span.hpp

diff --git a/include/rive/span.hpp b/include/rive/span.hpp
index bc86e3d..5dac1b5 100644
--- a/include/rive/span.hpp
+++ b/include/rive/span.hpp
@@ -9,6 +9,7 @@
 
 #include <initializer_list>
 #include <type_traits>
+#include <iterator>
 
 /*
  *  Span : cheap impl of std::span (which is C++20)