Make PixmapDirtyUpdateRec::src a DrawablePtr

This allows making the master screen's pixmap_dirty_list entries
explicitly reflect that we're now tracking the root window instead of
the screen pixmap, in order to allow Present page flipping on master
outputs while there are active slave outputs.

Define HAS_DIRTYTRACKING_DRAWABLE_SRC for drivers to check, but leave
HAS_DIRTYTRACKING_ROTATION defined as well to make things slightly
easier for drivers.

Reviewed-by: Adam Jackson <ajax@redhat.com>

include/pixmapstr.h

@@ -88,7 +88,8 @@ typedef struct _Pixmap {
 } PixmapRec;
 
 typedef struct _PixmapDirtyUpdate {
-    PixmapPtr src, slave_dst;
+    DrawablePtr src;            /* Root window / shared pixmap */
+    PixmapPtr slave_dst;        /* Shared / scanout pixmap */
     int x, y;
     DamagePtr damage;
     struct xorg_list ent;