Add a new directory tests/ for unit test applications. This directory
will be built only if --enable-tests is given to ./configure.

Add matrix-test application. It excercises especially the
weston_matrix_invert() and weston_matrix_inverse_transform() functions.
It has one test for correctness and precision, and other tests for
measuring the speed of various matrix operations.

For the record, the correctness test prints:

a random matrix:
   1.112418e-02   2.628150e+00   8.205844e+02  -1.147526e-04
   4.943677e-04  -1.117819e-04  -9.158849e-06   3.678122e-02
   7.915063e-03  -3.093254e-04  -4.376583e+02   3.424706e-02
  -2.504038e+02   2.481788e+03  -7.545445e+01   1.752909e-03

The matrix multiplied by its inverse, error:
   0.000000e+00  -0.000000e+00  -0.000000e+00  -0.000000e+00
   0.000000e+00   0.000000e+00   0.000000e+00   0.000000e+00
  -0.000000e+00  -0.000000e+00   0.000000e+00  -0.000000e+00
   0.000000e+00   0.000000e+00   0.000000e+00   0.000000e+00
max abs error: 0, original determinant 11595.2

Running a test loop for 10 seconds...
test fail, det: -0.00464805, error sup: inf
test fail, det: -0.0424053, error sup: 1.30787e-06
test fail, det: 5.15191, error sup: 1.15956e-06
tests: 6791767 ok, 1 not invertible but ok, 3 failed.
Total: 6791771 iterations.

These results are expected with the current precision thresholds in
src/matrix.c and tests/matrix-test.c. The random number generator is
seeded with a constant, so the random numbers should be the same on
every run. Machine speed and scheduling affect how many iterations are
run.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>