Implement GetMultismaplefvANGLE and SampleMaskiANGLE of texture multisample extension

This patch adds GetMultismaplefvANGLE and SampleMaskiANGLE api.

TEST=TextureMultisampleTest.CheckSamplePositions*
TEST=NegativeTextureMultisampleTest.Negtive*

BUG=angleproject:2275

Change-Id: Ib1e8b856059229bbe88328a28cf2a892f4673214
Reviewed-on: https://chromium-review.googlesource.com/c/1319252
Commit-Queue: Yizhou Jiang <yizhou.jiang@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/Context.cpp b/src/libANGLE/Context.cpp
index 1f660de..ef1f0c5 100644
--- a/src/libANGLE/Context.cpp
+++ b/src/libANGLE/Context.cpp
@@ -7843,6 +7843,7 @@
             case GL_MAX_INTEGER_SAMPLES_ANGLE:
             case GL_MAX_DEPTH_TEXTURE_SAMPLES_ANGLE:
             case GL_TEXTURE_BINDING_2D_MULTISAMPLE_ANGLE:
+            case GL_MAX_SAMPLE_MASK_WORDS:
                 *type      = GL_INT;
                 *numParams = 1;
                 return true;