anv: Default PointSize to 1.0 if not written by the shader

anv: Default PointSize to 1.0 if not written by the shader

The Vulkan rules for point size are a bit whacky.  If you only have a
vertex shader and you use points, then you must write PointSize in your
vertex shader.  If you have a geometry or tessellation shader, then it's
dependent on the shaderTessellationAndGeometryPointSize device feature.
From the Vulkan 1.0.38 specification:

   "shaderTessellationAndGeometryPointSize indicates whether the
   PointSize built-in decoration is available in the tessellation
   control, tessellation evaluation, and geometry shader stages. If this
   feature is not enabled, members decorated with the PointSize built-in
   decoration must not be read from or written to and all points written
   from a tessellation or geometry shader will have a size of 1.0. This
   also indicates whether shader modules can declare the
   TessellationPointSize capability for tessellation control and
   evaluation shaders, or if the shader modules can declare the
   GeometryPointSize capability for geometry shaders. An implementation
   supporting this feature must also support one or both of the
   tessellationShader or geometryShader features."

In other words, if the feature is disbled (the client can disable
features!) then they don't write PointSize and we provide a 1.0 default
but if the feature is enabled, they do write PointSize and we use the
one they wrote in the shader.  There are at least two valid ways we can
implement this:

 1) Track whether or not shaderTessellationAndGeometryPointSize is
    enabled and set the 3DSTATE_SF bits based on that and what stages
    are enabled, ignoring the shader source.

 2) Just look at the last geometry stage VUE map and see if they wrote
    PointSize and set the 3DSTATE_SF accordingly.

The second solution is the easiest and the most robust against invalid
usage of the Vulkan API, so we choose to go with that one.

This fixes all of the dEQP-VK.tessellation.primitive_discard.*point_mode
tests.  The tests are also broken because they unconditionally enable
shaderTessellationAndGeometryPointSize if it's supported by the
implementation and then don't write PointSize in the evaluation shader.
However, since this is the "robust against invalid API usage" solution,
the tests happily pass. :-)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>