Make MAX_DRAW_BUFFERS and MAX_COLOR_ATTACHMENTS equal

There's no benefit from having more color attachments than draw buffers,
or vice versa, since the highest-numbered color attachment that is
accessible for rendering is COLOR_ATTACHMENT<min(MAX_DRAW_BUFFERS,
MAX_COLOR_ATTACHMENTS) - 1>. This clarifies the API, like a similar
addition in the WEBGL_draw_buffers spec.