Validate ANGLE_multiview end-points

The patch adds validation for the ANGLE_multiview end-points
and propagates the calls to the unimplemented stubs in Context.

The patch also removes the
glFramebufferTextureMultiviewSideBySideRobustANGLE end-point
because the size of the viewport offsets buffer can trivially
be computed from the number of views.

BUG=angleproject:2062

Change-Id: I9a10bc00c19825e586d3df2750fabea4daf5ef8f
Reviewed-on: https://chromium-review.googlesource.com/573861
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/Context.cpp b/src/libANGLE/Context.cpp
index 35dcce6..7489e44 100644
--- a/src/libANGLE/Context.cpp
+++ b/src/libANGLE/Context.cpp
@@ -2653,6 +2653,8 @@
         mExtensions.colorBufferFloat      = false;
         mExtensions.eglImageExternalEssl3 = false;
         mExtensions.textureNorm16         = false;
+        mExtensions.multiview             = false;
+        mExtensions.maxViews              = 1u;
     }
 
     if (getClientVersion() > Version(2, 0))
@@ -3094,6 +3096,26 @@
     mGLState.setObjectDirty(target);
 }
 
+void Context::framebufferTextureMultiviewLayeredANGLE(GLenum target,
+                                                      GLenum attachment,
+                                                      GLuint texture,
+                                                      GLint level,
+                                                      GLint baseViewIndex,
+                                                      GLsizei numViews)
+{
+    UNIMPLEMENTED();
+}
+
+void Context::framebufferTextureMultiviewSideBySideANGLE(GLenum target,
+                                                         GLenum attachment,
+                                                         GLuint texture,
+                                                         GLint level,
+                                                         GLsizei numViews,
+                                                         const GLint *viewportOffsets)
+{
+    UNIMPLEMENTED();
+}
+
 void Context::drawBuffers(GLsizei n, const GLenum *bufs)
 {
     Framebuffer *framebuffer = mGLState.getDrawFramebuffer();