The GL_MESA_ycbcr_texture extension is broken
Yesterday I have been playing with GL_MESA_ycbcr_texture on my laptop with ATI RV530 GPU running Mesa 10.3.2 and wanted to report that UNSIGNED_SHORT_8_8_MESA is much slower (4 FPS instead of 36 FPS) than UNSIGNED_SHORT_8_8_REV_MESA because find_supported_format returns the first supported format instead of the one that best matches the data type. _mesa_swap2 being implemented as a C routine that swaps the bytes one pixel at a time also doesn't really help to achieve good performance.
Looking at the code I also doubt that it will work on big endian machines. It is unfortunate that the format is defined as one short instead of two bytes per pixel.
Anyway, I then wanted to check if the performance problem still exists with Mesa 20.1.0 and to my surprise the GL_MESA_ycbcr_texture extension is completely broken there. The first problem one runs into is that b112e62b inverted one condition so that GL_YCBCR_MESA is immediately rejected. But if this is fixed, compute_texture_format_swizzle dies in an assert because it doesn't know how to handle that format.