Data in RenderPassDesc is packed better to make room for resolve
attachments (9 bits made available; 8 for color and 1 for
depth/stencil).

mColorAttachmentRange contains values in [0, 8).  The right side of the
range is made inclusive (i.e. [0, 7]) so it will fit in 3 bits.

The number of samples is always a power of 2, and below 128 (for the
foreseeable future), so its log is stored instead in 3 bits.

This change doesn't add support for resolve attachments yet, but is
split off the one that does to simplify it.

Bug: angleproject:4836
Change-Id: I2856286d5239499d0ab0c78b8154881c003309bc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2306515
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>