This patch intends to implement glSampleMaski on D3D11 backends.

In D3D11 the sample mask word can be set by OMSetRenderState. This
function can only accept an UINT as its parameter SampleMask, so
the value MAX_SAMPLE_MASK_WORDS in D3D11 backends is set to 1.

BUG=angleproject:1592
TEST=dEQP-GLES31.functional.state_query.*.sample_mask*
TEST=dEQP-GLES31.functional.texture.multisample.*.sample_mask*

Change-Id: I27f509cf81624d95948b06739bb9fe21288d2d31
Reviewed-on: https://chromium-review.googlesource.com/691259
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>