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This patch implements a heuristics, which enables backing store for some
windows. When backing store is enabled for a window, the window gets a
backing pixmap (via automatic redirection provided by composite extension).
It acts a bit similar to ShadowFB, but for individual windows.

The advantage of backing store is that we can avoid "expose event -> redraw"
animated trail in the exposed area when dragging another window on top of it.
Dragging windows becomes much smoother and faster.

But the disadvantage of backing store is the same as for ShadowFB. That's a
loss of precious RAM, extra buffer copy when somebody tries to update window
content, potentially skip of some frames on fast animation (they just do
not reach screen). Also hardware accelerated scrolling does not currently
work for the windows with backing store enabled.

We try to make the best use of backing store by enabling backing store for
all the windows that are direct children of root, except the one which has
keyboard focus (either directly or via one of its children). In practice this
heuristics seems to provide nearly perfect results:
 1) dragging windows is fast and smooth.
 2) the top level window with the keyboard focus (typically the application
    that a user is working with) is G2D accelerated and does not suffer from
    any intermediate buffer copy overhead.
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

The modern desktops may use ARGB cursors. As the current
sunxi display controller support code can't handle this
type of cursor yet, the X server fallbacks to a software
cursor which is not visible under layers and ruining user
experience.

This patch adds empty implementations for "UseHWCursorARGB"
and "LoadCursorARGB" functions which just return error for
now (so that the X server still fallbacks to software cursor).
However we also introduce callback functions responsible for
notifying the DRI2 code about enabling/disabling the use of
hardware cursor. So that now hardware overlays are disabled
when switching to software cursor and re-enabled again when
switching back to hardware cursor.
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

Hardware cursor is necessary because it is also visible on
top of sunxi disp layers, while software cursor is not.

FIXME: there is one minor problem with negative cursor
positions. The hardware does not support them, so such
positions are just set to 0 for now. In the future this
can be solved better by changing the cursor picture and
showing only the parts which are visible on screen.