Add a variation of test-fallback-surface that forces the use of a 16-bit
pixman format code instead of the standard 32-bit types. This creates an
image surface akin to the fallbacks used with low bit-depth xservers.
Eliminate the need for a runtime test on the sizeof the private structures
by performing the check at compile time. This was provoked by Ginn Chenn
noting that the test was including a private header.
Jeff Muizelaar found a regression in commit 005436 and submitted this
little test to exercise it. The essence of the bug appears to be wrt to
the product of the CTM and device transform matrices.
