Implement pager scrollback history buffer

For now, use big fixed-size buffer that is lost on resize

kitty/data-types.h

@@ -173,11 +173,19 @@ typedef struct {
     line_attrs_type *line_attrs;
 } HistoryBufSegment;
 
+typedef struct {
+    index_type bufsize;
+    Py_UCS4 *buffer;
+    index_type start, end;
+    index_type bufend;
+} PagerHistoryBuf;
+
 typedef struct {
     PyObject_HEAD
 
     index_type xnum, ynum, num_segments;
-    HistoryBufSegment* segments;
+    HistoryBufSegment *segments;
+    PagerHistoryBuf pagerhist;
     Line *line;
     index_type start_of_data, count;
 } HistoryBuf;