raster image of struct-image-03-t

This test verifies the support for gamma correction of displayed PNG images. Several different images are displayed one above the other; if gamma correction is correctly performed based on the values in the gAMA chunk in the PNG file, the resulting displayed values are the same in all of the files (except for rounding error, which gives some artefacts at the right side of the lowest two images due to the very high levels of gamma correction needed for this test).