ANGLE D3D11 uses a geometry shader to expand points into quads.
This led to an edge case with gl_PointCoord. When the user references
gl_PointCoord in the fragment shader but renders with GL_TRIANGLES or
other non-point primitives, gl_PointCoord is undefined, but ANGLE
would produce a link error. This would break some very odd edge-case
shaders (e.g. a shader that is used with both points and triangles).

We can fix this by simply adding a dummy PointCoord value to our
vertex shader is all cases. If the user renders points, we ignore the
PointCoord value passed to the geometry shader. If they render tris
or lines and use PointCoord, the shader signatures will match, and
PointCoord will even have a sane value of (0.5, 0.5).

BUG=angleproject:1380

Change-Id: I322155cd3801d0241cabc9bb639a5aaa502831b3
Reviewed-on: https://chromium-review.googlesource.com/362779
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>