Per resolution of WebGL working group, added previously-removed
IMPLEMENTATION_COLOR_READ_FORMAT and IMPLEMENTATION_COLOR_READ_TYPE to
latest WebGL 1.0 spec. Updated earlier conformance suites to allow these
constants to be present, and top of tree conformance suite to require them
to be present.
Merge pull request #520 from kenrussell/implementation-color-read-type-format

Per resolution of WebGL working group, added previously-removed