It seems reasonable to validate the equality between stencilWriteMask
and stencilBackWriteMask only by comparing the effective bits.
An existing dEQP test[1] and a WebGL test[2] also use the max size of
stencil bits for checking the stencil mask values.

[1] StencilWriteMaskSeparateTestCase (gles3/functional/
es3fIntegerStateQueryTests.cpp)
[2] https://www.khronos.org/registry/webgl/sdk/tests/conformance/state/
gl-get-calls.html (Search for "minimumRequiredStencilMask ")

Change-Id: I68cefbe8a9f69c5a9ebcb30027a05581e3024add
Reviewed-on: https://chromium-review.googlesource.com/302703Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jinyoung Hur <hurims@gmail.com>
Commit-Queue: Jinyoung Hur <hurims@gmail.com>
Tryjob-Request: Jamie Madill <jmadill@chromium.org>