[dri2] Add tail pointer to reemitDrawableInfo callback.

[dri2] Add tail pointer to reemitDrawableInfo callback.

When the DRI doesn't parse the event buffer for a while, the X server
may overwrite data that the driver didn't get a chance to look at.  The
reemitDrawableInfo callback requests that the X server reemit all info
for the specified drawable.  To make use of this, the drive needs to know
the new tail pointer so it know where to start reading from.