The test generates a mipmap by "copying" a client-array of pixels to
miplevel 0 with glDrawPixels, then calling glGenerateMipmap. The test
expects the rendered level 0 to match the client-array source. Dithering
during glDrawPixels inteferes with that expectation.

Fixes on Intel Skylake:
  spec.ext_framebuffer_object.getteximage-formats init-by-clear-and-render
  spec.ext_framebuffer_object.getteximage-formats init-by-render

Regressed by Mesa:
  commit c4b87f129eb036c9615df3adcc1cebd9df10fc84
  Author: Chad Versace <chadversary@chromium.org>
  Subject: meta: Disable dithering during glGenerateMipmap

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99266
Cc: Mark Janes <mark.a.janes@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>