Add test for EXT_srgb.
Merge pull request #571 from jdashg/add-srgb-test

Add test for EXT_srgb.
Fixed bugs and used more WebGLTestUtils primitives in ext-sRGB.html test.

Verified on Firefox.
Merge pull request #595 from kenrussell/cleanup-srgb-test

Fixed bugs and used more WebGLTestUtils primitives in ext-sRGB.html test...