V3d point coord

In the cases where the FS doesn't consume the implicit line and point varyings, V3D 4.x hardware can be programmed to not emit them.

The driver was always emitting these when rendering points or lines, but it seems we only ever consumed the point varyings (point_x, point_y). This means that we can safely disable these varyings if we are rendering lines, whereas if we are rendering points we need to check if we are consuming gl_PointCoord or if there is any input enabled in the point sprite mask.

I was considering whether we need to add anything to the fs_key for this, but since it already stores the point sprite mask I think we don't need to do anything specific. Piglit and CTS seem to be happy too.