glx/dri2: use uint64_t instead of double to represent time for FPS calculation

glx/dri2: use uint64_t instead of double to represent time for FPS calculation

Wine or a windows app changes fpucw to 0x7f, causing doubles to be equivalent
to floats, which broke the calculation of FPS.
We should be very careful about using doubles in Mesa.

Henri Verbeet adds:
  For reference, this is done by for example d3d9 when a D3D device is
  created without D3DCREATE_FPU_PRESERVE set. In the general case
  applications can do all kinds of terrible things to the FPU control
  word of course.