mesa: implement GetMultisamplefv

mesa: implement GetMultisamplefv

Actual sample locations deferred to a driverfunc since only the driver
really knows where they will be.

V2: - pass the draw buffer to the driverfunc; don't fallback to pixel
      center if driverfunc is missing.
    - rename GetSampleLocation to GetSamplePosition
    - invert y sample position for winsys FBOs, at Paul's suggestion

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>