test: Add cover-test v5

test: Add cover-test v5

This test aims to verify both numerical correctness and the honouring of
array bounds for scaled plots (both nearest-neighbour and bilinear) at or
close to the boundary conditions for applicability of "cover" type fast paths
and iter fetch routines.

It has a secondary purpose: by setting the env var EXACT (to any value) it
will only test plots that are exactly on the boundary condition. This makes
it possible to ensure that "cover" routines are being used to the maximum,
although this requires the use of a debugger or code instrumentation to
verify.

Changes in v4:

  Check the fence page size and skip the test if it is too large. Since
  we need to deal with pixman_fixed_t coordinates that go beyond the
  real image width, make the page size limit 16 kB. A 32 kB or larger
  page size would cause an a8 image width to be 32k or more, which is no
  longer representable in pixman_fixed_t.

  Use a shorthand variable 'filter' in test_cover().

  Whitespace adjustments.

Changes in v5:

  Skip if fenced memory is not supported. Do you know of any such
  platform?

Signed-off-by: Ben Avison <bavison@riscosopen.org>
[Pekka: changes in v4 and v5]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Ben Avison <bavison@riscosopen.org>
Acked-by: Oded Gabbay <oded.gabbay@gmail.com>