Proto: Add preferred compression message and constants.

When accessing a virtual desktop from different devices, some may have
different image compression requirements, e.g. slow devices may prefer
the faster LZ4 over GLZ. This message instructs the server to switch the
image compression algorithm. This patch also promotes the
SPICE_IMAGE_COMPRESS_* constants so that they are available from both
the server and the client.

common/client_marshallers.h

@@ -72,6 +72,7 @@ typedef struct {
     void (*msgc_smartcard_reader_add)(SpiceMarshaller *m, VSCMsgReaderAdd *msg);
 #endif
     void (*msgc_port_event)(SpiceMarshaller *m, SpiceMsgcPortEvent *msg);
+    void (*msgc_display_preferred_compression)(SpiceMarshaller *m, SpiceMsgcDisplayPreferredCompression *msg);
 } SpiceMessageMarshallers;
 
 SpiceMessageMarshallers *spice_message_marshallers_get(void);

common/messages.h

@@ -630,6 +630,10 @@ typedef struct SpiceMsgcPortEvent {
     uint8_t event;
 } SpiceMsgcPortEvent;
 
+typedef struct SpiceMsgcDisplayPreferredCompression {
+    uint8_t image_compression;
+} SpiceMsgcDisplayPreferredCompression;
+
 SPICE_END_DECLS
 
 #endif /* _H_SPICE_PROTOCOL */

spice.proto

@@ -361,6 +361,17 @@ enum8 image_type {
     LZ4,
 };
 
+enum8 image_compress {
+    INVALID  = 0,
+    OFF,
+    AUTO_GLZ,
+    AUTO_LZ,
+    QUIC,
+    GLZ,
+    LZ,
+    LZ4,
+};
+
 flags8 image_flags {
     CACHE_ME,
     HIGH_BITS_SET,
@@ -922,6 +933,10 @@ channel DisplayChannel : BaseChannel {
         int32 last_frame_delay;
         uint32 audio_delay;
     } stream_report;
+
+    message {
+        uint8 image_compression;
+    } preferred_compression;
 };
 
 flags16 keyboard_modifier_flags {