Fixed glSampleCoverage signature (vi to vii)

The signature for glSampleCoverage was incorrect (vi); glSampleCoverage takes two arguments (vii).

See: https://www.khronos.org/registry/webgl/specs/1.0.2/#5.14.3
and http://www.khronos.org/opengles/sdk/docs/man/xhtml/glSampleCoverage.xml

See issue kripken/emscripten/issues/1890