Fix SIMD unsafe use of floating point.

Fix SIMD unsafe use of floating point.

This commit fixes unsafe simd / floating point interactions arising
from the current hybrid and 16x16 transform implementation.
These led to a raft of bugs and issues when the project was
built using VS2008 for Win32 though they did not show up with
the unix builds.

Gerrit makes a meal out of presenting the fix but all I have actually
done is indent the body of each function that uses floating point by
one level and bracket with emms instructions using  the function
vp8_clear_system_state(). See below.

function () {
  vp8_clear_system_state();
  {
  ... function body
  }
  vp8_clear_system_state();
}

This is almost certainly over the top in terms of number of emms
instructions but is a temporary measure pending implementation of
integer variants of each function to replace the floating point.

Limited testing suggests that this fixes the problems that arose for
Win32 VS2008 when the hybrid or 16x16 transforms were enabled.

Change-Id: I7c9a72bd79315246ed880578dec51e2b7c178442