piglit-fbo: Add new variables to track multiple color attachments

New variables added here stays unused. They'll be used by the
following patches in this series.
piglit-fbo: Enable the tests to create FBO with multiple color attachments
Remove the usage of attach_texture from piglit tests

and use the new method of specifying type of color attachments.

Earlier:
config.attach_texture = true;

Now:
config.num_rb_attachments = 0;
config.num_tex_attachments = 1;

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Add test to verify blitting to multiple render targets

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>