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

Still seems like we're playing fast and loose with integer types:
  * age is uint32 but never needs to be anything larger than a uint.
  * width and height are 'int' but gbm.h uses uint32_t for them.

Please consider:
  (1) Removing the "32" from MirBufferExtAge unless the implementation is explicitly limited to 32-bits.
  (2) Converting width and height from int to uint32_t (since that's what gbm.h uses?).

We've all been guilty of choosing wrong integer types in the past, but if we're in the business of redesigning existing interfaces then at least the new design should be more correct.