Add VNC Hextile decoding

Implements RFC 6143 SS7.7.4: rectangles announced as Hextile (type 5)
tile the update into 16x16 blocks, each either raw pixels or a
background fill plus an optional list of foreground/individually-
colored subrects, with background/foreground persisting across tiles
within one rectangle when not re-specified.

The pure byte-decode logic (tile metadata, subrect list) lives in new
src/vnc_pixel_codecs.h/.cpp, kept separate from
VncSessionBackend's wire-sequencing state machine so it's unit-testable
without a socket -- the pattern the plan calls for continuing into the
ZRLE/Tight work still ahead. Adds 5 fake-server tests covering a raw
tile, a background-only solid fill, uncoloured and individually-colored
subrects, and a 4-tile rectangle proving background persistence and
correct tile-cursor wraparound.

Verified against the live TightVNC test server (connect, frame,
cursor, clipboard all still work); that particular server always
chose Raw for the actual framebuffer content during this session, so
Hextile's real-world path isn't independently confirmed live -- the
unit tests are the primary correctness evidence for this phase.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/vnc_session_backend.h

@@ -6,6 +6,8 @@
 #include <QAbstractSocket>
 #include <QByteArray>
 #include <QImage>
+#include <QRect>
+#include <QRgb>
 
 class QTcpSocket;
 
@@ -20,10 +22,11 @@ class QTcpSocket;
 //
 // Scope (see plan / issue #3 for the full rationale): standard VNC
 // Authentication (security type 2) and no-auth (type 1) only -- not
-// Apple's Screen Sharing scheme (type 30). Raw + CopyRect encodings only.
-// No dynamic resize. Clipboard sync (Latin-1 only, per RFB's
-// ServerCutText/ClientCutText) and remote cursor shape sync (the Cursor
-// pseudo-encoding) are supported.
+// Apple's Screen Sharing scheme (type 30). Raw + CopyRect + Hextile
+// encodings (ZRLE/Tight compression not yet implemented). No dynamic
+// resize. Clipboard sync (Latin-1 only, per RFB's ServerCutText/
+// ClientCutText) and remote cursor shape sync (the Cursor pseudo-encoding)
+// are supported.
 class VncSessionBackend : public SessionBackend
 {
     Q_OBJECT
@@ -83,6 +86,10 @@ private:
         WaitingRawPixelData,
         WaitingCopyRectSource,
         WaitingCursorPixelData,
+        WaitingHextileTileSubencoding,
+        WaitingHextileTileMeta,
+        WaitingHextileSubrectData,
+        WaitingHextileRawTileData,
         WaitingSetColourMapHeader,
         WaitingSetColourMapData,
         WaitingServerCutTextHeader,
@@ -120,6 +127,18 @@ private:
     int m_lastPointerX;
     int m_lastPointerY;
 
+    // Hextile decode state (RFC 6143 SS7.7.4): tile-cursor position relative
+    // to the current rectangle's origin, plus the background/foreground
+    // colors, which persist across tiles within one rectangle whenever a
+    // tile doesn't re-specify them.
+    int m_hextileTileX;
+    int m_hextileTileY;
+    QRgb m_hextileBackground;
+    QRgb m_hextileForeground;
+    quint8 m_hextileSubencoding;
+    int m_hextileSubrectsRemaining;
+    bool m_hextileSubrectsColoured;
+
     void setState(SessionState state, const QString& message);
     void resetProtocolState();
     void processReceiveBuffer();
@@ -134,6 +153,8 @@ private:
     void sendPointerEvent();
     void sendWheelClick(quint8 wheelBit);
     void sendClientCutText(const QString& text);
+    QRect currentHextileTileRect() const;
+    void advanceHextileTile();
 };
 
 #endif