If the draw path is taken for resolve, the internal shader previously
calculated (1+sum(sample.a))/sampleCount instead of
sum(sample.a)/sampleCount.  Additionally, due to a typo, the result was
rounded.

This change also fixes a number of off-by-one errors when calculating
src/dst offsets in blit/resolve path.

Tests are added to resolve from default to FBO and vice versa to
exercise the draw path which was otherwise never invoked for color
buffers.

Bug: angleproject:4746
BUg: angleproject:4092
Change-Id: I4b9c181339b89af44b27d61d27a6b3d88cde2eea
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2288224
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Ian Elliott <ianelliott@google.com>