Expose: Use Render SetPictureTransform rather than resizing Window Pixmap to get decent performances.

Before each for each Window Pixmap, an scaled Image was created on the
client-side and then push back to the server. This was obviously really slow
as everything could be done server-side. SHM could have been used but there is
no point as Render handles efficiently scaling.

For now, when the Expose plugin is enabled, the whole screen is repainted,
which will be fixed in later commits.
Expose: Picture is only stored for a mapped Window and a Pixmap is required to create it.

This fixes unmapped Windows not being painted.
Cosmetic: Use existing inline function to calculate Window height/width including borders.
Expose: Border is scaled as well, so it should be 0. Also, scaled Window size must include border.
Expose: Render Extension allocate a PictTransform each time SetPictureTransform is issued.