vc4: clamp gl_PointSize to a minimum of 1.0

vc4: clamp gl_PointSize to a minimum of 1.0

The OpenGL ES spec requires that the value of gl_PointSize is clamped
to an implementation-dependent range matching what is advertised by
GL_ALIASED_POINT_SIZE_RANGE. For VC4 this is [1.0, 512.0], but the
hardware won't clamp to the minimum side of the range and won't render
points with a size strictly smaller than 1.0 either, so we need to
clamp manually. For points larger than the maximum size of the range
the hardware clamps automatically.

Fixes piglit test:
spec/!opengl 2.0/vs-point_size-zero

Reviewed-by: Eric Anholt <eric@anholt.net>