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tests: add yuv-buffer test

Pekka Paalanen requested to merge pq/weston:mr/yuvtest into master

This adds a test to ensure that the wl_shm formats YUV420, NV12 and YUYV are decoded and converted to RGB correctly in GL-renderer.

The test deliberately uses a 256 x 256 test image so that effects from width vs. pitch vs. stride cannot be observed, and row padding is zero. Also padding between planes is zero. Attempting to use a test image with less "round" dimensions lead to stride mismatch in GL-renderer, likely due to GL_UNPACK_ALIGNMENT being left at value 4. It is unclear if YUV wl_shm buffers' row stride needs to be aligned to 4 bytes or not, so I did not pursue fixing it. GL-renderer seems to be confusing width, pitch and stride even further, and not e.g. allow padding with ARGB buffers. See also: #354

Furthermore, the test arranges so that each 2x2 pixel block has the same color. This avoids having to consider chroma siting when sub-sampling. This way all the test cases can use the same reference image.

The source image chocolate-cake.png is taken and copyright by Pekka Paalanen, hereby licensed as http://creativecommons.org/licenses/by-sa/4.0/ .

I am submitting this with only wl_shm tests for now, because I don't think I can get the dmabuf tests done before holidays.

Cc @vitalyp

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