[cairo] Bug in _cairo_xlib_surface_create_similar

[Robert O'Callahan]

I recently discovered that cairo_create_similar on a 16-bit xlib surface
creates a surface backed by a 24-bit pixmap. This results in absymal
performance --- I can actually watch a copy from the similar to the
original move down the screen, on my dual Xeon, NVidia vendor driver
machine. It also means xlib-based code like GTK themes can't render to
the surface because it doesn't match the screen depth.

The attached patch makes _cairo_xlib_surface_create_similar match the
characteristics of the input surface as closely as I know how, and fixes
my issues. Please evaluate...

Rob
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