Add gui.screenshot Lua function.

lua/screenshot.cpp

@@ -0,0 +1,17 @@
+#include "lua-int.hpp"
+#include "framebuffer.hpp"
+
+namespace
+{
+	function_ptr_luafun lua_gui_screenshot("gui.screenshot", [](lua_State* LS, const std::string& fname) -> int {
+		std::string fn = get_string_argument(LS, 1, fname.c_str());
+		try {
+			framebuffer.save_png(fn);
+			return 0;
+		} catch(std::exception& e) {
+			lua_pushstring(LS, e.what());
+			lua_error(LS);
+			return 0;
+		}
+	});
+}

manual.lyx

@@ -1964,6 +1964,15 @@ Request subframe updates (calling on_paint() on subframes) to happen (on=true)
  Can be used anywhere.
 \end_layout
 
+\begin_layout Subsubsection
+gui.screenshot(string filename)
+\end_layout
+
+\begin_layout Standard
+Write PNG screenshot of the current frame (no drawings) to specified file.
+ Can be used anywhere.s
+\end_layout
+
 \begin_layout Subsection
 table input
 \end_layout