This variable tracks the index range of color attachments.  A previous
change (d8ce865b) made the range
inclusive so it can fit in 3 bits, but that's incorrect as it's no
longer possible to specify a range where no color attachments are
present.

This change partially reverts d8ce865b
to restore the previous semantics of
RenderPassDesc::mColorAttachmentRange.

Bug: chromium:1107884
Change-Id: I08da2568b5e63a48a672edd499a8b6653060eadb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2310578Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>