Testing direct image layer optimisation

# Image optimisation in layers

This test exercises direct compositing of images with hardware acceleration. The visual results using ACCELERATED_COMPOSITING and regular TOT should be identical. Running this test manually with the correct debug options will show which elements are directly composited. See https://bugs.webkit.org/show_bug.cgi?id=23361

Basic image - no style - can be directly composited
5px blue border - can NOT be directly composited
margin - can NOT be directly composited
solid background - can be directly composited
background image - can NOT be directly composited
rotated but otherwise no style - can be directly composited