vnc: allow fall back to RAW encoding

I have observed that depending on the contents and the encoding it happens
that sending data as RAW sometimes would take less space than the encoded data.
This is especially the case for small updates or areas with high color images.
If sending RAW encoded data is beneficial allow a fall back to RAW encoding
for the framebuffer update.
Signed-off-by: NPeter Lieven <pl@kamp.de>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

ui/vnc.c

@@ -840,6 +840,8 @@ int vnc_raw_send_framebuffer_update(VncState *vs, int x, int y, int w, int h)
 int vnc_send_framebuffer_update(VncState *vs, int x, int y, int w, int h)
 {
     int n = 0;
+    bool encode_raw = false;
+    size_t saved_offs = vs->output.offset;
 
     switch(vs->vnc_encoding) {
         case VNC_ENCODING_ZLIB:
@@ -862,10 +864,24 @@ int vnc_send_framebuffer_update(VncState *vs, int x, int y, int w, int h)
             n = vnc_zywrle_send_framebuffer_update(vs, x, y, w, h);
             break;
         default:
-            vnc_framebuffer_update(vs, x, y, w, h, VNC_ENCODING_RAW);
-            n = vnc_raw_send_framebuffer_update(vs, x, y, w, h);
+            encode_raw = true;
             break;
     }
+
+    /* If the client has the same pixel format as our internal buffer and
+     * a RAW encoding would need less space fall back to RAW encoding to
+     * save bandwidth and processing power in the client. */
+    if (!encode_raw && vs->write_pixels == vnc_write_pixels_copy &&
+        12 + h * w * VNC_SERVER_FB_BYTES <= (vs->output.offset - saved_offs)) {
+        vs->output.offset = saved_offs;
+        encode_raw = true;
+    }
+
+    if (encode_raw) {
+        vnc_framebuffer_update(vs, x, y, w, h, VNC_ENCODING_RAW);
+        n = vnc_raw_send_framebuffer_update(vs, x, y, w, h);
+    }
+
     return n;
 }