mesa: Rename MESA_shader_integer_mix to EXT_shader_integer_mix

mesa: Rename MESA_shader_integer_mix to EXT_shader_integer_mix

Everyone at the Khronos meeting was as surprised that GLSL didn't
already support this as we were.  Several vendors said they'd ship it,
but there didn't seem to be enough interest to put in the effort to make
it ARB or KHR.

v2: Fix a couple typos and rename the spec file to
EXT_shader_integer_mix.spec.  Suggested by Roland.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>