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