mesa: Allow querying the system FBO in GetFramebufferAttachmentParameteriv

mesa: Allow querying the system FBO in GetFramebufferAttachmentParameteriv

        If the default framebuffer is bound to <target>, then
        <attachment> must be one of FRONT_LEFT, FRONT_RIGHT, BACK_LEFT,
        BACK_RIGHT, AUXi, DEPTH_BUFFER, or STENCIL_BUFFER, identifying a
        color buffer, the depth buffer, or the stencil buffer, and
        <pname> may be FRAMEBUFFER_ATTACHMENT_OBJECT_TYPE or
        FRAMEBUFFER_ATTACHMENT_OBJECT_NAME.

as well as these <pname> values

        FRAMEBUFFER_ATTACHMENT_RED_SIZE,
        FRAMEBUFFER_ATTACHMENT_GREEN_SIZE,
        FRAMEBUFFER_ATTACHMENT_BLUE_SIZE,
        FRAMEBUFFER_ATTACHMENT_ALPHA_SIZE,
        FRAMEBUFFER_ATTACHMENT_DEPTH_SIZE,
        FRAMEBUFFER_ATTACHMENT_STENCIL_SIZE,
        FRAMEBUFFER_ATTACHMENT_COMPONENT_TYPE, or
        FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING.

https://bugs.freedesktop.org/show_bug.cgi?id=28551