draw the root pixmap, and bind the glxpixmap to the texture only once

draw the root pixmap, and bind the glxpixmap to the texture only once

NOTE: derek please check my use of glTexEnvi (it seems to be working but idk)
root pixmap comes from _XROOTPMAP_ID and ESETROOT_PMAP_ID
the glxpixmap only needs to be bound to the texture once after the glxpixmap
 is created, so do that rather than every time we draw the texture.
added create_glx_pixmap(), bind_glx_pixmap(), and destroy_glx_pixmap()
 functions in composite.c to help wth this process
added a "bound" variable to ObWindow that tracks if the glxpixmap was
 successfully bound to the texture, so when we destroy the glxpixmap, we can
 tell if we should unbind/release it from the texture