The argument to glReadBuffer should be validated against
GL_MAX_COLOR_ATTACHMENTS, not GL_MAX_DRAW_BUFFERS.

Bug: angleproject:4766
Change-Id: Ic184934bd01e68b936266eae69543ae66fd68ea3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2252978Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Kenneth Russell <kbr@chromium.org>