Code review comment for lp://staging/~kdub/mir/gbm-ext-v2

(7) I should have picked a different function in (6) because I have another question...
  typedef uint32_t (*MirBufferGbmStride)(MirBuffer* buffer);
Why isn't there a generic mir_buffer_get_stride() function already?

(8) Does it make sense for a buffer's age to be queried without any context?;
  typedef unsigned int (*MirBufferGbmAge)(MirBuffer* buffer)
You may be using a buffer in two or more different rendering contexts so it can have a different age for each. I don't think MirBufferGbmAge() makes sense.