Added conformance tests to check using WebGL canvases as texture sources

These tests currently pass on Firefox and Safari but fail completely on Chrome. IE passes about 1/4th of the new test, but already has issues with the other related texture tests so this isn't exposing a new class of failures for them.

Thanks to Gaëtan Renaudeau for pointing out the broken behavior.
Minor nit: the opt_fontTest code path is unused and incomplete in this test and should probably be removed.