videotestsrc: Various improvements
Replace moving-color-bars pattern with smpte100, and change moving-speed to horizontal-speed. Default is now 0. Add a rotation stage to pattern building. Allocate a temporary scanline for building images. Remove unused code. Disable several patterns that we're unable to test and probably never used. Add other variants of bayer sampling. Convert some patterns to use videotestsrc_blend_line.