Function write_layout_prim_blocks 

pub fn write_layout_prim_blocks(
    builder: &mut GpuBufferBuilderImpl<GpuBufferBlockF>,
    prim_rect: &LayoutRect,
    clip_rect: &LayoutRect,
    pattern_base_color: ColorF,
    pattern_texture_input: RenderTaskId,
    segments: &[QuadSegment],
) -> GpuBufferAddress

Write the gpu blocks for a primitive in layout space.