mesa: Fix regression introduced by commit "mesa: fix packing of float texels to GL_SH...

mesa: Fix regression introduced by commit "mesa: fix packing of float texels to GL_SHORT/GL_BYTE".

This commit "mesa: fix packing of float texels to GL_SHORT/GL_BYTE" replaced *_TO_BYTE to *_TO_BYTE_TEX because *_TO_FLOAT_TEX are used to unpack the texels to floats.
In this case *_TO_FLOATZ in function extract_float_rgba also should be replaced to *_TO_FLOAT_TEX. Underline that these macros automatically preserve zero when converting.

The regression was observed on 3 oglconform tests:
    snorm-textures basic.getTexImage
    snorm-textures advanced.mipmap.manual.getTex
    snorm-textures advanced.mipmap.upload.getTex

Signed-off-by: Pavel Popov <pavel.e.popov@intel.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>