Validate the ReadPixels type and format for INVALID_ENUM.
Validate the ReadPixels type and format for INVALID_ENUM.

GL_INVALID_ENUM should be generated if the type or format arguments were
not valid for any ReadPixels command.  This validation should happen
before validating if the combination of format and type is valid.

TEST=conformance/reading/read-pixels-test

BUG=angleproject:2000

Change-Id: If49d69655f5ab0a3abbde58b49541c84249c1750

ppy/angle,ppy/angle,MSOpenTech/angle,ecoal95/angle,ecoal95/angle,ppy/angle,ecoal95/angle,ppy/angle,MSOpenTech/angle,MSOpenTech/angle,ecoal95/angle,ecoal95/angle,MSOpenTech/angle