Add image flag for "all high bits are set to one"

This lets us send rgba images with all high bytes 0xff as rgb, which
is not all that uncommon.

spice/draw.h

@@ -171,6 +171,7 @@ enum {
 
 enum {
     SPICE_IMAGE_FLAGS_CACHE_ME = (1 << 0),
+    SPICE_IMAGE_FLAGS_HIGH_BITS_SET = (1 << 1),
 };
 
 typedef struct SPICE_ATTR_PACKED SpiceImageDescriptor {

spice/qxl_dev.h

@@ -410,6 +410,7 @@ typedef struct SPICE_ATTR_PACKED QXLImageID {
 
 enum {
     QXL_IMAGE_CACHE = (1 << 0),
+    QXL_IMAGE_HIGH_BITS_SET = (1 << 1),
 };
 
 enum {