image: make getDrawBufferImage() work for ES or !ARB_draw_buffers

Without this 'apitrace dump-images' was failing for me (GLES, freedreno
gallium driver) because attachment would be GL_NONE.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Carl Worth <cworth@cworth.org>
Signed-off-by: José Fonseca <jfonseca@vmware.com>
glstate: Cleanup the read buffer selection code slightly.

We were calling glReadBuffer(draw_buffer) before reading the GL_READ_BUFFER.