[vte] emulation: sixel: Implement DECSET 1070

[Christian Persch <chpe src gnome org>]

commit 4b2c688dde8b239ec2af1e7d709893a328737e5f
Author: Christian Persch <chpe src gnome org>
Date:   Fri Oct 23 20:16:55 2020 +0200

    emulation: sixel: Implement DECSET 1070
    
    Private mode 1070 controls whether each SIXEL image gets a freshly initialised
    set of colour registers, or if changes from one SIXEL image persist to the next.

 src/modes-private.hh | 13 ++++++++++++-
 src/modes.hh         |  5 ++++-
 src/vte.cc           | 10 ++++++++++
 src/vteinternal.hh   |  1 +
 src/vteseq.cc        |  4 +++-
 5 files changed, 30 insertions(+), 3 deletions(-)
---
diff --git a/src/modes-private.hh b/src/modes-private.hh
index 255a968e..6bd0ca9b 100644
--- a/src/modes-private.hh
+++ b/src/modes-private.hh
@@ -163,6 +163,18 @@ MODE(XTERM_SAVE_CURSOR,              1048)
 MODE(XTERM_OPT_ALTBUF_SAVE_CURSOR,   1049)
 MODE(XTERM_READLINE_BRACKETED_PASTE, 2004)
 
+/*
+ * XTERM_SIXEL_PRIVATE_COLOR_REGISTERS:
+ * When set, each SIXEL graphic uses newly initialised colour registers.
+ * When reset, changes to colour registers from one SIXEL image are
+ * saved and used for the next SIXEL graphic.
+ *
+ * Default: set
+ *
+ * References: XTERM
+ */
+MODE(XTERM_SIXEL_PRIVATE_COLOR_REGISTERS, 1070)
+
 /* Not supported modes: */
 
 /* DEC */
@@ -858,7 +870,6 @@ MODE_FIXED(XTERM_FKEYS_HP,                      1052, ALWAYS_RESET)
 MODE_FIXED(XTERM_FKEYS_SCO,                     1053, ALWAYS_RESET)
 MODE_FIXED(XTERM_FKEYS_LEGACY,                  1060, ALWAYS_RESET)
 MODE_FIXED(XTERM_FKEYS_VT220,                   1061, ALWAYS_RESET)
-MODE_FIXED(XTERM_SIXEL_PRIVATE_COLOR_REGISTERS, 1070, ALWAYS_SET)
 MODE_FIXED(XTERM_READLINE_BUTTON1_MOVE_POINT,   2001, ALWAYS_RESET)
 MODE_FIXED(XTERM_READLINE_BUTTON2_MOVE_POINT,   2002, ALWAYS_RESET)
 MODE_FIXED(XTERM_READLINE_DBLBUTTON3_DELETE,    2003, ALWAYS_RESET)
diff --git a/src/modes.hh b/src/modes.hh
index 398155de..9ddf7429 100644
--- a/src/modes.hh
+++ b/src/modes.hh
@@ -239,7 +239,10 @@ public:
                                    eDECSDM,
                                    eVTE_BIDI_SWAP_ARROW_KEYS,
                                    eXTERM_ALTBUF_SCROLL,
-                                   eXTERM_META_SENDS_ESCAPE} { }
+                                   eXTERM_META_SENDS_ESCAPE,
+                                   eXTERM_SIXEL_PRIVATE_COLOR_REGISTERS}
+        {
+        }
 
         inline void push_saved(int mode)
         {
diff --git a/src/vte.cc b/src/vte.cc
index 855ca454..2a5c8bd6 100644
--- a/src/vte.cc
+++ b/src/vte.cc
@@ -9812,6 +9812,15 @@ Terminal::reset_data_syntax()
         pop_data_syntax();
 }
 
+void
+Terminal::reset_graphics_color_registers()
+{
+#ifdef WITH_SIXEL
+        if (m_sixel_context)
+                m_sixel_context->reset_colors();
+#endif
+}
+
 /*
  * Terminal::reset:
  * @clear_tabstops: whether to reset tabstops
@@ -9936,6 +9945,7 @@ Terminal::reset(bool clear_tabstops,
          * clear (assign to black) all SIXEL colour registers.
          * (DEC PPLV2 § 5.8)
          */
+        reset_graphics_color_registers();
 }
 
 void
diff --git a/src/vteinternal.hh b/src/vteinternal.hh
index 7fb233c6..57895de3 100644
--- a/src/vteinternal.hh
+++ b/src/vteinternal.hh
@@ -951,6 +951,7 @@ public:
         void im_reset();
         void im_update_cursor();
 
+        void reset_graphics_color_registers();
         void reset(bool clear_tabstops,
                    bool clear_history,
                    bool from_api = false);
diff --git a/src/vteseq.cc b/src/vteseq.cc
index 461ef3c8..ecdc6974 100644
--- a/src/vteseq.cc
+++ b/src/vteseq.cc
@@ -478,7 +478,7 @@ Terminal::update_mouse_protocol() noexcept
 
 void
 Terminal::set_mode_private(int mode,
-                                     bool set) noexcept
+                           bool set) noexcept
 {
         /* Pre actions */
         switch (mode) {
@@ -4118,6 +4118,8 @@ Terminal::DECSCL(vte::parser::Sequence const& seq)
                 break;
         }
 #endif
+
+        reset_graphics_color_registers();
 }
 
 void