meta: Don't do fragment color clamping in _mesa_meta_pbo_GetTexSubImage

meta: Don't do fragment color clamping in _mesa_meta_pbo_GetTexSubImage

_mesa_meta_pbo_GetTexSubImage() uses _mesa_meta_BlitFrameBuffer(),
which will do fragment clamping if enabled. But fragment clamping
doesn't affect ReadPixels and GetTexImage.

Without this patch, piglit test arb_color_buffer_float-clear fails,
when forced to use the meta pbo path.

v2: Apply this fix to both glReadPixels and glGetTexImage.

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>